Hi Luke,
What do you mean by "just boot from the microSD"?
The SABRE Lite essentially does that. What I mean is that all boot instructions are on the card in the boot script (6x_bootscript).
We have boot scripts that load Windows Embedded, Android, QNX, Yocto, Timesys, Ubuntu, Debian, et cetera and switch between them simply swapping cards.
You can also force the boot loader to load from SD, but this is a tricky operation, and recommended only for 'actual experts'...