I have started a sourdough using sprouted buckwheat grains. Dehulled grains sprout just fine, provided they aren't roasted (Russian-style wholegrain buckwheat is sold roasted, at least where I live).
First wash/rinse the grains, twice in cold and once in warm water. Soak 20-30 mins, then strain. I leave mine in a strainer, covered with a cloth, and it sprouts overnight.
Put the sprouts and a little water in a blender, mix into a slurry. Salt. This is your starter
! I have only made a soft flat bread (
rieska in Finnish) by mixing soaked roasted buckwheat into the sourdough and letting it sit for 8-12 hours, but I think you'll do fine if you want to use rice or buckwheat flour to make a thicker dough.
I have not kept a starter of a previous batch, as the sprouted grains sour beautifully every time. The taste of the bread much resembles the traditional sour rye bread I grew up with.