I created an ISO image file with the new slides (and as a backup, the edited photos I used in it) to burn to a fresh CD. I handed the .iso off to 'hdiutil burn', expecting it to eject the drive and prompt me for a blank CD.
The CD-R I'd borrowed had not been closed, so hdiutil happily interpreted my command as a request to burn this new .iso as another session on the disc it already had, and not in append mode. Dropping the CD into the Mac or the Windows box now only revealed my new volume. The program book data was now inaccessible. Apple's Disk Utility wouldn't show the old session, either. I think I recall an INIT for OS 9 that would mount all sessions of a CD, but I don't have time to track one down (if it exists) for OS X or Win2000.
Fortunately, I'm not Completely Fucking Stupid.
mount -t iso9660 -o ro,session=0 /dev/scd0 /cdrom1on the Linux box mounted the original session, allowing me to pull the data off and to burn fresh copies of it.