If you create a patch which can be applied on all of your master's builders, you can modify your Buildbot master so the patch is applied (as in the TryMaster configuration).
cd /work/chromium && patch -p0 < /tmp/patch
Place the patch at /tmp/patch (this path should be readable by your Buildbot master).
Modify ./scripts/master/chromium_step.py with the following patch:
Index: chromium_step.py
===================================================================
--- chromium_step.py (revision 27647)
+++ chromium_step.py (working copy)
@@ -58,7 +58,10 @@
args = copy.copy(self.args)
args[‘revision’] = revision
args[‘branch’] = branch
cmd = buildstep.LoggedRemoteCommand(‘gclient’, args)
self.startCommand(cmd, warnings)