Moving unbundled builder to svn:externals

I just updated my project to the latest edge revision (7774) and I
discovered that Builder has been unbundled “in favor of gem
dependency”. I did install the builder gem and was able to get my app
back up and running, but I was wondering if there was a way to use an
svn:externals property to download builder to the vendor directory
instead of using gems. I usually do this so I can run multiple apps
on the same box at different points in the rails trunk.

I know there is a way to freeze gems, but I prefer not to have all of
those changes duped in my repository and instead let the changes exits
in the originating projects repo. Any ideas how to achieve this with
the recently unbundled Builder?