Multiple OS installer from USB

I once had an external USB HDD with installers fro Mac OS X 10.5 Leopard, Mac OS X 10.6 Snow Leopard, Mac OS X 10.7 Lion and OS X 10.8 Mountain Lion and it was easy to create.
Just partition the USB flash storage thumb drive via Disk Utility with as many partitions as you like, use GUID as partition scheme and either use Disk Utility or CarbonCopyCloner (version 3.4.7 is still free and available for download here and works with Mac OS X 10.6 Snow Leopard, Mac OS X 10.7 Lion and OS X 10.8 Mountain Lion) to clone or extract the original installers onto the partitions.

I used CCC and the images I made from the Mac OS X 10.5 Leopard and Mac OS X 10.6 Snow Leopard install DVD and the images for Mac OS X 10.7 Lion and OS X 10.8 Mountain Lion I purchased upon their release.

 

The HDD is probably faster, Mac OS X 10.6 Snow Leopard was installed in less than 15 minutes.
If you get a fast USB 3.0 flash storage thumb drive though, you can get close to saturating the USB 2.0 bus, and I recently purchased this USB 3.0 drive in order to reinstall Mac OS X 10.6 Snow Leopard onto my 2009 MBP via USB 2.0 and it took ten minutes or so to create the image onto it and the install took around 20 minutes, probably even less.

I went with a USB 3.0 flash drive in order to get a faster read and write speed, as most USB 2.0 flash drives are quite slow and around 5 to 10 MB/s.

 

I would recommend partitions no smaller than 8GB. I am actually using SDHC cards with partitions to do what you want to do. What I like about the SD cards is that they have a write protect switch to prevent accidents when you're not updating them.