Booting a USB device when unsupported by the BIOS

[Mark Morgan Lloyd]

For an older computer which doesn't boot from external (e.g. USB) 
devices but will load OFW from floppy or a sacrificial disc partition, 
and which is then able to see an external USB-connected CD-ROM drive, is 
there a generic command to tell it to boot from a CD (containing e.g. 
Linux or Windows) or would this rely too heavily on BIOS facilities that 
are unavailable once OFW is running?

Alternatively, given probe-usb output of

USB2 devices:
USB1 devices:
/pci/usb@7,2/cdrom@0,0
/pci/usb@7,2/cdrom@0,0/disk

how can I best look at the media to determine the name of kernel, initrd 
etc., and subsequently convert those into boot-device, boot-file, 
ramdisk etc. parameters? I've tried  dir /pci/usb@7,2/cdrom@0,0/disk@0:0 
  etc. but the disc spins for a while and then reports iso9660-file-system.

I'm trying to write this up to answer an FAQ on a private conferencing 
system I use.