Hey,
I don't know how many of you are using the Eclipse plugin for compiler work, but I just pushed a bash script that should make it a bit easier to use it with a custom built scala-compiler:
In principle, once you built your modified compiler (and are fairly confident it's binary compatible with the one you had installed in Eclipse), you can run
$ update-scala.sh --eclipse-dir /path/to/eclipse update build/pack/
(the old bundles are saved before modifying, so if something goes wrong you should be able to revert). This assumes your existing installation is a nightly based on 2.10 (since this script won't recompile all the IDE dependencies with your new compiler, obviously)
Hope this helps,