Well, for what its worth it will function in OSX.
What you can't do is boot from a partition with any drive connected through it. That's not usually a problem, and for a server, you'll likely want a seperate drive for system and apps than where you store your data.
I had a 45GB IBM external hooked up with a 2940U2W since the beta days on an unsupported 7600 with a G3 400mHz upgrade in it. Performed wonderfully with more than 320MB.
I'm now running YDL2.2 on this machine with the card, but it certainly works. By unsupported, it means they won't listen to you whine if it stops working with a future revision of the OS. As of 10.1.4 it was working fine (I installed YellowDog since). Just remember you *won't* want to try booting from a drive connected (which may work, I never tried it).