Aha! Running:
guix environment --ad-hoc --no-grafts racket -- drracket
launches DrRacket just fine.
My guess is that Racket CS is compressing string literals in compiled
code. Currently, Guix patches Racket source files to include the
absolute paths to foreign libraries in the store as string literals.
There are a bunch of grafts for GTK and such: if I'm right, Guix somehow
mangles the compiled code while attempting to apply the grafts.
I already thought this strategy was a bad idea. If it is really the
problem, I should be able to patch it fairly quickly: I've already been
experimenting along these lines.
-Philip