Foaming shouldn't be a problem because the only bubbles that can get in are exactly the same as the rate the engine uses fuel. In an ordinary tank set up where the vent (pressure) line goes to the top of the tank when upright then that vent goes to the bottom of the tank when you fly inverted and it then bubbles at exactly the same rate as a uniflow tank. In fact, with a normal vent like this, when you fly inverted the tank acts as a uniflow all the time the model is flying inverted. That's why most engines will lean out slightly when going inverted because the needle hasn't been tuned for a uniflow tank.