Gerrit,
I ran across this URL in my archives; perhaps it has some interesting/useful patches you can integrate as gtk rolls forward.
The neat thing is that it appears to include both win32-based and X11-based windowing methods, but I'm not sure if these backends conflict with each other. However, ALL are cygwin-based, and thus distinct from Tor Lilqvist's native MS distro.
http://web.sfc.keio.ac.jp/~s01397ms/cygwin/
(However, I DO NOT want to imply that you should delay your current work to go look at this stuff; if it is useful, I'm sure it can be integrated later).
-- Chuck
