This might be really awkward, but could you seed different kinds of platforms? They would all look and work exactly the same to the players, but they would be marked with a "maximum difficultly." I level 6 platform would always be there, and a level 3 platform would be removed on difficulties passed normal.
Since they players can only change difficulty inside of a settlement, you can be guaranteed that they can't change it while in a room. In multiplayer, you set it based on the first person into a room.
You could do the generation in multiple passes. Add in just the bare minimum, then add in a few more for each step down. Heck, level 6 could spawn none! Or, at least very, very few.