you look prepared for the battle, I no more fear for you to lose the sanity :):)

I have cassiopeia(MIPS) PocketPC 3.0, and also have some ARM-enabled device with PocketPC 4.20
When you'll be running 5.0, could you please look at "serious limitation" there? :)

As for threads - I did the build (so compiler did not choke), but actual threading was not working, rather all threads were waitning for other threads.
I am not optimistic for threading on Win32, so IMO WinCE even not worth trying... (your opinion may vary, of course :)

As for Win32::GUI - I think there are chances for it to work, albeit stripped version. Just comment out what will be not working, and remaining stuff will work. Actually WinCE C library is quite good at Win32 GDI, yet CELIB has some more.
Look at CELIB sources, there are many Win32-like functions.