On both our prior vessel, a Pearson Vanguard, and our current boat, a Westsail 42, we have understood that the documentation number needs to be "permanently" affixed to the vessel or hull. The key issue is what is meant by "permanent".
On our Vanguard, the prior owner did a masterful job of drilling the outline of the number into the maststep support beam with a 1/8 drill bit. It was a thing of beauty (not really), but was certainly permanent. On our WS42, the prior owner had the number ornately carved or routed into a piece of teak and bolted to the inside of the coaming at the nav station. This application is nice looking (really), and is fairly permanent.
I have seen the routed number on a board affixed to the hull in an engine room in a lot of boats. In the Pardey's Talesin they have their number carved into the inside of the cabin side at the conmpanionway/bridge deck location. You can see it in many of their videos. If a companionway dropboard is permanent enough then it would look very similar to how the Pardeys have theirs.
Doug