With git/mercurial, I've used submodules/subrepos with plugins "installed" as a local plugin. I think you can do the same sort of thing with svn externals, but I'm guessing it wouldn't be as clean.
The nice thing about this method is that it avoids the whole compile, package, deploy to maven repo steps as well as all the snapshot versioning stuff. Makes things much quicker.
It might not scale that well though if you've got a _lot_ of internal plugins that you're using, but for a few that you're doing lots of iteration on, this can be a huge time saver.
Publish the stable plugins and use subrepos/internal plugins for the things you change often.
-Ted