Well, I can tell you the following: I accidently flashed my Mac/PC HD3870 card with the PC-only firmware ... yes, I mixed-up the two cards within my Mac Pro

However, I was able to reflash it with the Mac/PC rom and it now works within OSX as before. Conclusion: A HD3870 can be flashed with the Mac/PC rom if the you are lucky enough to find a card with a big enough rom chip (128K). Unfortunately the Sapphire HD3870 is 99% identical to the Mac/PC version but has a 64K chip

I am not aware of any case somebody identified a 128K rom chip PC HD3870 card so far ... soldering might be one alternative