To follow up: I gave up on NetBSD on this machine and moved my routing
to a Soekris box. I recently attempted to install OS X on it again and
was having the same booting issues. I think I managed to fix this by
fiddling with the DIP switches on my G3 upgrade card (NewerTech), and by
replacing the battery. This is after I replaced the SCSI cable and RAM,
both of which did nothing.

If anyone experiences the same issue, try setting the NewerTech card to
use a 45mhz bus instead of 50mhz. It doesn't seem to impact performance
in any noticeable fashion, and seems to have fixed what may be a timing
issue. I'm pretty sure this'd apply to NetBSD if I were to try that
again.

Thanks for all the help.
-Claudio