I've just gotten a SyQuest EZ135 drive, and I now wonder how feasible the
following idea would be:  If the d_packname field were filled in with
something unique, you could use something like the automount daemon to
pretend that the disk was permanently mounted, at least in a
half-hearted way:  upon trapping a reference to the disk, the modified
amd would prompt on the console (ugh) to mount the desired volume, and
could either poll the drive until the volume appears, or have some
explicit notification means (either run a program that communicates
with amd, or maybe just have it read "OK\n" from the console (bleah)).
The difficulty, though, is that only one such volume could be in use
as once, because you wouldn't be able to dismount a volume in the
drive while a file on it was still open (amd gets itself out of the
way after the first reference, right?  Maybe it could hang around and
continue to intercede, so that the only direct user of the actual
filesystem would be the modified amd itself). 

What I'm driving at, for those who've seen it, is something like the
ability of the Mac to request floppies and other removable media on
demand.  While those demands sometimes become irritating :-), it's
more convenient than having to catalog media in a text file.