A great software solution. I had a look and once Linux boots it actually uses devicemapper to map a dm device to the raw sectors containing the iso which it extracts (as it may be fragmented) to feed to dmsetup. Kinda crazy.

There is also a couple of hardware projects that to that in hardware. I used the ISOstick for many years.

You can also PXE NetBoot a lot of installers with dynamic menu selection - see http://netboot.xyz