Yes, you can boot your N810 from mmc1 - I am right now.

IIRC the instructions are the same as for the N800 except you will need to make a bootmenu.conf file before flashing due to a bug with the N810s if a SD card (mmc2) is inserted and you're booting from mmc1. Do a search for bootmenu.conf and you can find some examples of how to fix or for and more info. Fanoush includes some samples on the flasher zip.