Well, ONE thing that will work is:
1. Partition the USB stick into several parts
2. Unpack the contents of each *.iso into each part
3. Install GRUB in one partition (you may modify one of the partitions with an iso image for this purpose)
4. Configure GRUB to allow you to start whichever installer you wish.
Hmm... well, you'll have to try it to see if it works - I can't guarantee that the installer's init scripts don't attempt to mount and chroot to a nonexistent CD. Then again, even if they did, that just means a little extra work unpacking the initrd images and modifying their 'init' scripts. The only drawback is that you can't just change your boot/install options by copying yet another *.iso file to a single directory. Also, the number of installers you can boot is somewhat limited by the number of partitions you can create.