Hello "CirclingTiger"
I had an old Palm III with an adb port but I of course needed an usb/serial adapter when I bought an IMac (long time ago 😉 running system 9. I had to use the pc craddle as there was no usb/adb but it worked fine and even later with this PowerMac and 10.1, as far I remind. But now I use usb, since I have a treo, but I put it back tonight and I can see it in Panther via USB Prober (a little tool installed with Develloper tools) : still recognized, so it's like thoses adapters could be ze zolution ?
Did you have a look at keyspan ?
http://www.keyspan.com/ They have a Mac OS Section with Tiger compatibility inside.
I am waiting for my MacBook pro (shipping the 05/31..) so I don't have Tiger neither a MacIntel at the moment to tell you more.
PowerMac G4 "Mirrored drive doors" Mac OS X (10.3.9)