It should still work. I was only stating that I'd tested it under OS X. I'm not totally sure what 9.1 has for USB chipset drivers, though, and it's possible you may only get USB 1.1 speeds out of it (OHCI driver only, perhaps).
That said, the only thing a 'Mac-specific' USB/1394 card has, that a PC one with the same chipset doesn't, is a ROM with Apple Fcode in it, allowing a USB keyboard to be used in OF or a 1394 disk to be bootable, or the like. As long as you don't need that, anything with the right chipset should work. As for what the right chipset consists of, I'm less sure for OS 9 than OS X, but NEC is a pretty good bet for USB (OHCI, and everyone who groks USB at all groks OHCI). TI is a good bet for 1394 both because all but the old PCILynx chips (and whatever the gimpy half-a-Firewire hootenanny in a VW540 is...) implement OHCI, and IIRC, Apple used TI chips in Powerbooks, but I could be misremembering.
It's worth Googling around to see if there's an EHCI driver for OS 9 floating around, too - that'd certainly come in handy.