Well, perhaps people are looking at the problem wrong? Maybe these z80 ASM experts working on the C->ASM compilers should try a totally radical technique in writing compilers. Considering z80 is 8-bit/psuedo 16-bit, it's probably difficult to include all the features of (32-bit) C. After working with TIGCC for awhile and comparing it to the little z80 knowledge I have, I've noticed that 32-bit programming seems considerably different from 8-bit/16-bit.
My suggestion is perhaps to write a C->ASM compiler, but also create an ASM optimizing program (kind of like source coder? am I right?). I don't know if it's possible though. Actually, ASM optimizing is somthing that could be very helpful for the community because it takes a lot of experience before one can write efficient and optimized ASM code that could power the engine of say, an FPS.