I agree with Richard that the main cause of "Wet Sumping" would be if the head of oil in the tank slowly leaked through the feed side of the pump and ended up in the sump.
A non-return valve fitted in the feed line would not stop this but if fitted the wrong way round would prevent any oil from ever getting into the engine.
The best solution would be to fit a tap in the feed line but if you forgot to turn it on before starting the engine the result would be disasterous and the engine would be wrecked very quickly.
If the "Wet Sumping" is caused by oil leaking past the scavenge side of the pump, in theory the volume of oil ending up in the sump would only be equal to the amount left in the return oil line back to the tank because the end of the return line is above the oil level in the tank.
I doubt this would be sufficient volume to cause any real problems.
