I'd be submitting a patch to mingw32 people as soon as it's done, in addition to posting it here. Note that the only way for me to test it would be to inspect the assembly output, as I'm not running 64 bit environment here (even though I'm on a 64 bit AMD processor). So it'd need testing from 64 bit people here at least.
I'm volunteering. I could also give you ssh access to a 64 bit environment.
Once I get sensible assembly coming out I'll let you know. I'll try the __attribute__(__msvccall__) for per-function, and -msvc for per-file.
Cheers, Kuba