Thanks for all your suggestions. Just a quick update: the issue, it
seems, was due to two bad memory sticks. The system didn't detect
them until I enabled the POST. The fact that a simple command like
"boot cdrom" caused a freeze should have incited me to try the POST
straight away. Well that's fixed.

BTW, DoN., my version of the Openboot PROM on this Ultra 30 seems to
support both 512 and 2048 byte blocks. Have now been able to boot
with both. Would mention the version number for posterity, but the
machine is at home. FWIW.