An update: I have managed to get the FA-101 (somewhat) working with the iBook.
I have misreported the version of Ableton Live that didn't work with the FA-101 in my previous post; it was 1.01. I have now tested 2.1.2, and after disabling virtual memory and moving the FA-101 ASIO driver to its "ASIO Drivers" folder, I'm happy to report that the FA-101 ASIO device is selectable in Live 2.1.2.
So far, I have been able to test basic audio output on two channels through the headphone output, and guitar and microphone recording through the two preamp inputs. They work as expected. However, with audio playing through Firewire, a typical Live set runs rather choppy on the iBook, despite only consuming around 20% CPU, and seems to grind to a halt a while later, with the controls gradually turning unresponsive and audio dropping out. Maybe an OS reinstall and some performance optimization will help, but maybe the 500mhz iBook with 320 MB of memory is simply under spec for Firewire operation. (I'm not sure, since the manual does not seem to state minimum specs for OS 9. In fact, it says nothing about OS 9 at all; all instructions for "driver installation" are based on OS X.)
Cubase VST 32 / 5 also works as expected, but again, there are dropouts.
With the Powerbook G4, Live 2.1.2 still fails to detect the FA-101. Now that I'm pretty much sure the FA-101 in my hands is not defective, since its basic functions work with the iBook, I'm beginning to suspect that this laptop may have a Firewire hardware problem. I will test again after reinstalling OS 9 on it. If that fails, it seems I'll have to get another (USB) audio interface for it.