Not sure if this would be helpful, but....
I used to have a Zaurus (well, 's still there but dead battery), which had rather limited internal flash memory. To enable users to
a) boot other OS without reflashing,
b) overcome the limit of the built-in memory, and
c) to protect the internal memory from wearing out
they used pivot_root to swap the root fs during boot up to one supplied on a e.g. SD card.
The man page of pivot_root explains the basic procedure...

PS: no N900 here yet, otherwise would have tried it myself.