I think I might not have made myself clear - given an execution trace of a test suite with full coverage (or just something which exercises every code path - it doesn't have to be a test suite, but you've already got one of those so you might as well use it, right?), you could build a tool to make a rename refactoring fully automatic in a dynamic language. At the point of use, it would be identical to a static language rename, it would just work in a different way. It's a fairly obvious idea, so I presume this has been done in the past, but I don't know of any examples offhand.

The only gotcha is where you're eval'ing a string, but that's going to be nasty however you cut it.