Or can you assign miner to a builder standing on steel? That would be questionable behavior by itself.
You cannot. However, the inconsistency here is that you
can assign them to a lemming (builder or not) standing on a wrong direction one-way-wall. The result being a single swing that doesn't destroy the terrain below (of course, it might destroy terrain above the lemming if there's any and it's not also one-way), followed immediately by the lemming reverting to a walker and turning around. I believe that, apart from the lack of any terrain removal, this is identical to L1 behaviour.