new file mode 100644
--- /dev/null
+++ b/hgext/manualoverlay/__init__.py
@@ -0,0 +1,130 @@
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+import os
+import re
+import urllib
+
+from mercurial.i18n import _
+from mercurial import (
+    cmdutil,
+    commands,
+    demandimport,
+    encoding,
+    error,
+    extensions,
+)
+
+OUR_DIR = os.path.normpath(os.path.dirname(__file__))
+execfile(os.path.join(OUR_DIR, '..', 'bootstrap.py'))
+
+# requests doesn't like lazy importing
+with demandimport.deactivated():
+    import requests  # noqa
+
+
+testedwith = '4.1 4.2'
+
+cmdtable = {}
+command = cmdutil.command(cmdtable)
+
+REVISION_KEY = 'subtree_revision'
+SOURCE_KEY = 'subtree_source'
+LINEAR_REPO_URL = 'https://hg.mozilla.org/projects/converted-servo-linear'
+GITHUB_PR_URL = re.compile(
+    r'https://github\.com/servo/servo/pull/(?P<pr>[1-9][0-9]*)/?')
+COMMIT_MSG_PR = re.compile(r'servo: Merge #(?P<pr>[1-9][0-9]*) - .*')
+
+
+def commitcommand(orig, ui, repo, *args, **kwargs):
+    pr_url = kwargs.get('manualservosync')
+    if not pr_url:
+        return orig(ui, repo, *args, **kwargs)
+
+    m = GITHUB_PR_URL.match(pr_url)
+    if m is None:
+        raise error.Abort(
+            _('--manualservosync was not a proper github pull request url'),
+            hint=_('url must be to a servo/servo pull request of the form '
+                   'https://github.com/servo/servo/pull/<pr-number>'))
+
+    pr = m.group('pr')
+
+    revset = urllib.quote('keyword("servo: Merge #%s")' % pr)
+    url = '%s/json-log?rev=%s' % (LINEAR_REPO_URL, revset)
+    r = requests.get(url)
+    commits = r.json()['entries']
+
+    if not commits:
+        raise error.Abort(
+            _('could not find linearized commit corresponding to %s' % pr_url),
+            hint=_('If this pull requests has recently been merged it '
+                   'may not be linearized yet, please try again soon'))
+
+    repo.manualsync_commit = {
+        'desc': encoding.tolocal(commits[0]['desc'].encode('utf-8')),
+        'node': encoding.tolocal(commits[0]['node'].encode('utf-8')),
+        'user': encoding.tolocal(commits[0]['user'].encode('utf-8')),
+    }
+    repo.manualsync_pr = pr
+    repo.manualsync = True
+    return orig(ui, repo, *args, **kwargs)
+
+
+def reposetup(ui, repo):
+    if not repo.local():
+        return
+
+    class servosyncrepo(repo.__class__):
+        def commit(self, *args, **kwargs):
+            if not self.manualsync:
+                return super(servosyncrepo, self).commit(*args, **kwargs)
+
+            # Override some of the commit meta data.
+            msg = self.manualsync_commit['desc']
+            user = self.manualsync_commit['user']
+
+            # This method has many keyword arguments that mercurial
+            # ocassionally passes positionally, meanig they end up
+            # in *args, instead of **kwargs. This can be problematic as
+            # naively modifying the value in **kwargs will result in
+            # the argument being passed twice, which is an error.
+            # Protect against this by stripping the values out of
+            # *args and **kwargs, passing them positionally ourselves.
+            for key in ('text', 'user'):
+                if args:
+                    args = args[1:]
+
+                if key in kwargs:
+                    del kwargs[key]
+
+            kwargs['extra'] = kwargs['extra'] if 'extra' in kwargs else {}
+            kwargs['extra'][SOURCE_KEY] = encoding.tolocal(LINEAR_REPO_URL)
+            kwargs['extra'][REVISION_KEY] = self.manualsync_commit['node']
+
+            # TODO: Verify that the file changes being committed are only
+            # under the servo/ directory.
+            ret = super(servosyncrepo, self).commit(
+                msg, user, *args, **kwargs)
+
+            ctx = repo[ret]
+
+            if any(not f.startswith('servo/') for f in ctx.files()):
+                self.ui.warn(
+                    _('warning: this commit touches files outside the servo '
+                      'directory and would be rejected by the server\n'))
+
+            return ctx
+
+    repo.__class__ = servosyncrepo
+    repo.manualsync = False
+
+
+def extsetup(ui):
+    entry = extensions.wrapcommand(commands.table, 'commit', commitcommand)
+    options = entry[1]
+    options.append(
+        ('', 'manualservosync', '',
+         'manually overlay and sync a servo pull request', _("GH_PR_URL")))

new file mode 100644
--- /dev/null
+++ b/hgext/manualoverlay/tests/helpers.sh
@@ -0,0 +1,14 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+export HGUSER='Test User <someone@example.com>'
+
+cat >> $HGRCPATH << EOF
+[extensions]
+strip =
+manualoverlay = $TESTDIR/hgext/manualoverlay
+
+[diff]
+git = true
+EOF

new file mode 100644
--- /dev/null
+++ b/hgext/manualoverlay/tests/test-pr-url-validation.t
@@ -0,0 +1,40 @@
+  $ . $TESTDIR/hgext/manualoverlay/tests/helpers.sh
+
+  $ hg init source
+  $ cd source
+  $ echo "file1" > file1
+  $ hg add file1
+  $ hg commit -m "Initial commit"
+  $ echo "updated-file1" > file1
+
+Test passing a non-url
+  $ hg commit --manualservosync "not-a-well-formed-url"
+  abort: --manualservosync was not a proper github pull request url
+  (url must be to a servo/servo pull request of the form https://github.com/servo/servo/pull/<pr-number>)
+  [255]
+
+Test passing a url to a non servo/servo pull request
+  $ hg commit --manualservosync "https://github.com/wrong/repo/pull/17455"
+  abort: --manualservosync was not a proper github pull request url
+  (url must be to a servo/servo pull request of the form https://github.com/servo/servo/pull/<pr-number>)
+  [255]
+
+Test passing a url with an alphanumeric pull request id
+  $ hg commit --manualservosync "https://github.com/servo/servo/pull/abc123"
+  abort: --manualservosync was not a proper github pull request url
+  (url must be to a servo/servo pull request of the form https://github.com/servo/servo/pull/<pr-number>)
+  [255]
+
+Committing without --manualservosync should still work
+  $ hg commit -m "Normal Commit"
+  $ hg log -r .
+  changeset:   1:94650105917f
+  tag:         tip
+  user:        Test User <someone@example.com>
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     Normal Commit
+  
+
+Overlay extra data shouldn't exist on a normal commit
+  $ hg log -r . --template '{extras}'
+  branch=default (no-eol)

--- a/testing/vcttesting/testing.py
+++ b/testing/vcttesting/testing.py
@@ -34,16 +34,19 @@ PYTHON_COVERAGE_DIRS = (
 # by the Python test harness.
 UNIT_TEST_DIRS = {
     'autoland/tests': {
         'venvs': {'global'},
     },
     'git/tests': {
         'venvs': {'global'},
     },
+    'hgext/manualoverlay': {
+        'venvs': {'vcssync'},
+    },
     'hgext/overlay': {
         'venvs': {'vcssync'},
     },
     'hgserver/tests': {
         'venvs': {'global'},
     },
     'pylib': {
         'venvs': {'global', 'hgdev'},