> My wild guess is that the disk image would need a small ISO filesystem
> at the front to contain the "El Torito" boot information, which would
> just contain a traditional BSD boot sector; then the BSD disklabel (in
> the unused portion in front of the ISO filesystem) would describe a
> BSD 'a' partition which starts just after the ISO filesystem.  Is this
> right?

 Almost. mkisofs can assemble such a ISO image - you need to write
some additional information within the first 16 tracks of the CD.
Also, El Torito does not support "large" chunks of FFS, only floppy-sized 
images and (cannot look it up right now) small sized disk partitions. The 
disk partitions need to be available as a file during the image construction 
state (use 'dd' to do so). 

 "man mkisofs" should tell you more about the process.

-- 
Markus Illenseer