After realizing I had tried to absorb too much too fast in terms of McCLIM and backend implementation, and after taking a break from it, this weekend I pressed the Reset button. Not that I've thrown any code away, but rather, I'm going to spend some time working with McCLIM in its natural habitat -- that is to say, via SBCL on Linux and the CLX backend (maybe I'll experiment with gtkairo too, we'll see). I'll get some practical experience with it in an environment where it works, then return to my Win32 backend implementation.
[Update: well, my Gentoo install is old enough that GTK+ 2.8.19 fails to build. So it's back to CLX on Win32 after all. I really should upgrade my Linux, though.]