Perfectly legit maneuver. There are four guns defense maneuver taught by TOPGUN. Break turn, barrel-roll over the top, barrel-roll underneath and neg-G pushaway. The neg-G pushaway is exactly as the OP describes it. A hard push forward followed by a hard pull (usually out of plane). The push spoils the attackers gun solution and, if done starting from a high-G turn, will usually cause the attacker to lose sight. The out of plane positive G pull then serves to force an overshoot and yes, planes are perfectly capable of going from +6G to -2G and back to +6G again without damage. I agree with the others though that the neg-G is very uncomfortable and there are other practical limits.
First, most RL pilots never actually get to negative G in a push, they think they do but are actually just hitting 0G and starting to float in their seat but even that will sometimes work. You have really got to stuff the stick to get a true 1-2 negative G and it's very unnatural and uncomfortable at first so it takes practice. I always did a negative G check as part of my combat checklist to "warm up" and also to make sure that nothing would come flying up and hit me in the head. I've never seen anyone able to push and sustain sufficient negative G during a guns defense to cause any damage to themselves, it's too short of a duration.
The biggest problem isn't red-out, it's staying in your seat. For ACM you're supposed to fly with your lapbelts as tight as you can get them but the reality is most of us flew with the lap belts "comfortable". The shoulder restraints are on inertia reels and can be locked or left free so you can turn but the lap restraints are fixed. The problem is that you have to strike a balance between really strapping in and being able to turn around. This is particularly a problem if you're short and the cockpit is large. Lap belts on modern ejection seats don't do a great job of keeping you in your seat during negative G even when fully tightened because they are designed to pull back and down vice straight down. What happens during negative G is that you end up "hanging" upside down from the lap restraints and that lifts your feet off the pedals and makes it hard to keep forward pressure on the stick so the trick is to have your seat restraints tight enough to be able to maintain control without limiting your ability to turn too much. The newer seats now have a neg-G strap which comes up between your legs but I only found it to be a little bit better but it still didn't keep your butt in the seat.
As far as WWII aircraft are concerned, I imagine that there was a broad difference in the quality of their restraint systems. Hartman's stories leads me to believe the Germans must have had a pretty good system but it's hard to say.