There's no real boot framework on mobile devices the way there is for PCs. Android boxes have traditionally had the ability to boot into one of two kernel+initrd environments: "recovery" and "system", and that's it. The hardware OEMs make this framework work via whatever tricks they want to play, and then ship.

Obviously a second stage bootloader could be written, but it's going to be highly hardware-specific -- one for the OMAP4-based Galaxy Nexus wouldn't work with the Tegra3-based Nexus 7, etc...

Could you at least write a rebooter that would rewrite /boot and then have two "partitions"? You could go into Ubuntu and choose "reboot into Android" and it would rewrite /boot and then restart the device?

I wonder how the bootloader for the Touchpad works. It had the functionality I describe though I have no idea how it was implemented.

Yes, you could do that. On all the devices I've played with the kernel+initrd image is a standard format and exposed as a single partition. You'd just dd your new one over it and reboot. But this would be fragile, to say the least.

There's no real existing support for dual-booting on the N7 I'm aware of; and the effort to do so would likely be better spent on Ubuntu-ARM itself.

At least the Nexus 7 is a cheap platform. I'm almost tempted to get one if I can stay away from Android (and the related Google dependence/data leak).

The other advantage is that it could help people fully commit rather than switching backwards and forwards.