MultiROM is an amazing hack (I want to thank again its author, Tasssadar) that permits to boot different OS (multiboot) and different android roms. It starts between kernel and rom init, showing a sort of bootloader dialog to choose which system to boot.
It supports installing (and booting, of course) roms to USB drives.
To use MultiROM you need multirom installation zip , a modified TWRP recovery and kernels with kexec support. Relax, is not hard or long to do this.
More information and installation (of MultiROM, recovery and kernels) --> [ MULTIROM ]
PS: I need to specify I coded nothing, my only work was helping with issues, repacking roms, make them work. I want to say thank you to all devs are developing for Nexus 7, I'm doing this because I appreciate their work and I want MultiROM to rise. It deserves to.
HOW TO INSTALL A ROM.
Tasssadar provided also a zip template (see the [ WIKI ] for more info) that is incredibly easy to use.
To install a linux os to sdcard, you just need to download the zip and flash it like a normal update.zip.
It won't replace your internal rom, because it is going to install in /sdcard, not in /system or /data. And it WON'T wipe anything.
Remember that flashing ubuntu or android rom is DIFFERENT from flashing those zips. To flash ubuntu or another android rom you need to use MultiROM menu under ADVANCED voice in recovery (again, see wiki or multirom thread if you need more information).
In the next post I'll share links for rom zips. If you have other zips or if you have built other zips, please share them. They will be (hopefully) added to the list. If not, please pm me. I can't read all posts.
If you want to share a zip, please try it first. I don't know if I'll be able to test everything real-time, and I don't want anyone to see his sdcard erased or strange things like this. MultiROM has a percent of risk, but nobody has lost his data till now. So please share zips consciously.