You could always tie it to craftmesh and then bury the mesh, and let the rhizome hang out on the surface. I bought a medium piece of Mopani for my 5 gal, and just tucked the rhizome into the perfect little crevice in the wood. I tied the end on with fishing line so it wouldn't slip off.
There ARE nano-sized driftwood out there, you just have to look. I found my 5" "tree" on eBay, and a slew of listings for pieces of wood on Etsy, eBay, and plenty of Cholla/Choya on Aquabid (I just don't trust my frog not to somehow get himself stuck)