I'm not too concerned. I found a temporary workaround: to make a new block in Loot++ (a "placeholder block") using the same texture. Then I tell Loot++ that the stairs are based on the new placeholder block and it textures them properly.
Oh yeah, actually, it looks like it's rendering in the same place, but
the furnace is rendering on top of it. That could be because I made it
render in the alpha pass (so it's possible to add transparent furnaces).
Oh, ok. I'll have to play with my transparencies a bit more, I guess.