If your NAND (read rootfs) is okay, this may be possible.
It'll take quite a lot of hacking though.
There is a thread here somewhere on on booting to eMMC.
Use that but make it mount the SD card instead.
Additionally, modify some scripts in /etc/event.d to make them mount SD instead of eMMC as/under MyDocs.
All the above takes quite a bit of hacking, numerous reboots, etc.
I have some related stuff done already and can share but need you to figure things out.

Btw, booting frm eMMC, the N900 may feel more sluggish. A combo of rootfs in NAND and SD would be best.

kh