I taught a lot of aerobatics in the past. Enjoyed showing how to fly and recover from any situation by flying outside the envelope of normal airline flying. My first airline we even flew a B737 sim inverted over LAX just because it was fun. Not sure how the fuel would be picked up in a real 737 but the sim did just fine. My model airplane had a tube with a weight on the end of the fuel line to keep fuel flow while inverted. I kind of think Boeing didn't think that was necessary. The famous 707 roll at SEA when the test pilot showed what it could do was not a negative G roll or slow roll, it was more like a zero G aileron roll.