I think it should work, but is it worth it? If you can get a PC version of the Radeon 9000 Pro, which can easily be flashed if they are the ATI retail one that looks like this. They are found for like $50 and cheaper than these adapters, and this is for the 128 MB model.
Mac Elite has the ROM. You can flash the card on your Mac if you have a spare PCI card or using VNC on Mac OS X with Graphicellerator and moving both switches on the upper left-hand corner to SF. Of course, the Radeon 9000 Pro isn't the fastest card on Mac OS 9. It's the Nvidia Geforce 4 Ti 4600 (which is rare) and the Radeon 8500 for ATi, which is also easy to flash, but uncommon. Anything from 9500 or higher does not work on Mac OS 9. The Geforce 4 Ti 4600 are flashable with the Ti Rom Maker, which can be found on this forum. However, flashing it requires a PC with an AGP slot. Also, finding a working PC version can be tricky as they are rare. I tried buying one, but I got a defective one and had to send that back. I ended up with a Mac version, but not a big deal since I'm not using two monitors as I have one big 1920x1200 display, which is enough.