I cannot see how the standard system would work in any prolonged negative G or inverted flight.
If I remember right, somebody told me, could have been Rainer Wilke, that there are two small feeder tanks close to the engine which solve that problem.
I asked Rainer how he does his very tight rolls - not the barrel rolls - without loosing altitude, well he does, he is flying kind of an arc and is falling down. There is the zero g. The upside down pedal turn would also be zero g or low negative g (actually wouldn't that be high, since it is negative). And it never lasts for very long, since you can not have negative pitch.