Slightly off topic, but speaking of your kexec idea, may I suggest setting the flashable kernel's cmdline for boot device to:
so that the system will be completely independent of the state of Harmattan, Nemo and NITDroid?

The base system can probably be done with evtap + Harmattan components (as seen in the touch preinit)

With the right interface script, you can do everything.

It would allow really easy repartitioning and an extra layer of safety (no need to load Rescue Initrds anymore, you can simply not care about whether the eMMC gets wiped, and with modifiable boot menus, you can kexec into Harmattan outside of mmcblk0p2).

You can probably even slap a complete Debian base system into the "var" partition, which is ~500MB.

Oh, and I've answered some of your questions on the triple-boot thread.