Yes, yes, I did know roughly what I was doing.... Been doing this stuff since 1991.
It appears to be a scsi termination problem, though what problem precisely I have not yet determined — I have yet to complete the complete scsi voodoo ritual. I have, however, found just today that the unit will not recognize any (internal) scsi device on the scsi bus with my Wallstreet powerbook connected in scsi disk mode. Once the pb is disconnected, however, the internal device, which in this case is a newly installed zip drive, shows right up.
It may be some peculiarity with the G3AIO, or indeed with the Wallstreet, but on the basis of this, I'm sure I can get to the bottom of it eventually. My original query was basically whether installation of a firewire device effectively disables scsi disk mode in favour of a similar connection via firewire. It seems not.