So, an (ignorant) owner in our world wants to trade one of his ML players after the trade deadline, only he hasn't put the player through waivers yet.

Out of curiosity, I sent him an offer for that player (sending a minor leaguer in return), expecting the system to prevent me from even submitting the proposal. It actually went through and is waiting for his reply.

What actually happens here? Will HBD prevent the owner from accepting the trade? Or will the player have to clear waivers before the trade can be completed? And, if so, will that work as revocable waivers?

Thanks.

The trade will not go through if the player has not cleared waivers.  Doesn't matter if they are revocable waivers or not, though it's in the other owners best interests to put him on revocable waivers, and it's in your best interests to put in a claim for him once he hits the wire.