I've tested more: it definitely IS working in Tiger, complete with volume control in Prefs.
This is on a G4 iMac running 10.4.8 (with Pro Speakers) and on a G4 PowerBook running 10.4.7 (with internal speakers). Both systems have also made use of the iSub on earlier Tiger (and Panther) versions. I haven't even seen any quirks or unreliability on the PowerBook, but it's only been used there occasionally. On the iMac, which has been using the iSub for years, there is some quirkiness, including a tendency (ever since Panther) to get no sound after waking--but re-plugging the USB connector brings it back.
So, bottom line... not perfect in Tiger, but mostly working fine.
HOWEVER, on my Intel iMac (C2D 24" 10.4.8), I get nothing from it, and no controls appear in Prefs.
So it does seem to be an Intel OS X thing, and not an overall Tiger thing.
Regardless, an update would be very welcome! The iMac is where I really want to use it, to complement the internal speakers.