Well, I have an uneven floor on the first floor of an old Victorian building due to historical structural movement. I'm thinking of ways to fix this. Being a first floor there will be no sub-floor, just floorboards mounted on joists. I know the best solution is to just make a variety of wooden levelling shims.
However, I'm toying with the idea of using levelling compound in a slightly unorthodox way. the idea would be to dam the exposed joists either side with plywood or similar (and at either end), and use a levelling compound in the dam. I would then want to be able to screw it down to the joists and then relay the floorboards and screw through the floorboards, hardened compound and joists to fix them in place.
I didn't say this right away since everyone will say this is mad, but I'm just wondering about the possibility.