It might mean a bit of work, but look into Grub4Dos, you can use it to chainload the bootsectors of whatever media you want (no guarantees it will work), you might have to make some mods to the ISO's to get them to work 100%. I used to do this personally at work, cause I got fed up of people nicking all my CD's, but even this was a pain to setup and maintain!
if you want something a little different, I suggest looking at acquiring one of these (http://isostick.com/), the boot selector doesn't always work on certain devices, but you can manually set media to load via a text file, that option has never let me down!
As a final alternative, if you use an external HDD, the multiple partitions approach will work (pendrives only present the first partition to be bootable), but you'll still need a bootloader (EXTLINUX, Grub4Dos, Grub, etc)!