It shouldn't be harmful, but I don't believe that millipedes would appreciate it as much as other creatures. With the exception of Orthoporus, my millipedes seem to prefer being on or in the substrate. A lightweight half-log or curved piece of bark might work better. Many of my millipedes enjoy a piece of rotted wood they can chew on and burrow into.
I put a piece in the tank with my ivories, mostly to give them something to climb on. One ivory seems to like eating it so I left a small piece in there. I've now put it in with my gigas. One's shell is damaged and I fear it was from the pieces of wood I had in there being too heavy or something. So I pulled out everything remotely heavy but they want something to curl up under. I broke the log in half and provided two places for them. The one with the injured shell pretty much immediately moved under it. The other did after munching on some food.
I haven't gotten up to the point of getting some rotting wood I know is safe for them.