As far as I know, no promise cards can be made bootable. Adaptec cards have a slight-chance of being made bootable.
One of the main issues with flashing from PC to mac is the size of the ROM on the card. If it's below a certain size, the only way you can get it working if someone reverse-engineers the ROM and rewrites it and some how makes it smaller.
However, if you have the appropriate sized rom, go for it! I think you can flash it in a MAC using adaptec's utilities, but you may have better luck doing it with a PC due to not really being anything keeping you from doing it.
Adaptec made more money on the Mac SCSI Cards because they could charge more since it was a Niche market. One thing they did was kept their utility from flashing PC cards. So if someone has figured out how to get an adaptec ROM utility to flash ROMs without looking at the type of card, you will have luck.
It's just much more dead easier to flash it from a PC under DOS or something.
Go ahead and give us a try. However if you do, try to list the Revision of the card, Model number, and the chipset so we can do it ourselves (or have knowledge). I would be interested to see which cards you can flash from PC to Mac!