I just thought of something you could try. Reroute the vent line so that it ends at the bottom of the tank instead of at the top. This will probably mean using a different fill method but that's an easy fix. You'll need to retune your engine (it'll need to be richened a little) because now no matter how much fuel is in the tank the engine will think it's almost empty while that vent is covered with fuel. When you go inverted the vent line will be uncovered and then the mixture will feel the amount of fuel but because the tank (when inverted) is a bit low it should balance out quite well.

For those that are thinking "what the????"...it'a variation on the uniflow tank priciple. All normal RC clunk tanks become a uniflow when you fly inverted (the vent is then at the "bottom" of the tank) and the engine tends to lean out a little. This is just making it work the other way round.