How to handle tank slappers?

Since the term come from motorcycles I will give you the motorcycle answer...which likely will not apply on a MR car.

Counter steer (front wheel into the turn) and counter balance (lean out of the turn) and either power out of it or use only rear brake.

My EX500 race bike ended up in a tank slapper an this worked perfectly

What you are likely talking about is the massive left, right, left right tail wag from slide, overcorrect, slide opposite, overcorrecti, etc.  In my uneducated experience, for that car, turn into the slide and correct with mild application of throttle.  alternatively, go two feet in and just let it happen.

My experience in successful spin recovery, when I have been successful, has always been in my track day Miata, a model that is very forgiving and feels like it's self recovering at times. For me, it's been about getting weight on the back wheels. If it's under braking, modulating the brake pedal to get weight to the rear for bite while steering to get the front to follow the rear. If it's under power, feeding on the power to get weight to rear for bite while follow steering.

Not saving it is an option I tried once. Crank the wheel away from where the rear is going and jam on the brakes to get the car to spin in a tight circle and not crash-drift hit the approaching tire wall. Either through luck or intention, I spun past it. A friend was once in a crash-drift towards the outside tire wall at T8 at Sonoma. He did above technique to intentionally spin into the huge grass area on the inside. Got a big talking to by the HPDE safety steward but he didn't bend his car. His action was my inspiration for my intentional tight spin.

I wish there was a wet skidpad around the SF area where I could try practicing spin management in laboratory conditions.

In car.  I crested a hill, got over a little too far and it broke loose at the top over curbing/grass.  this was from a little too much driving in the rearview, as a bunch of fast cars were behind me.

Maybe 85/90mph.

Most of the fun starts at around 1:00.

https://www.youtube.com/watch?v=RxbnfSQ8R2s

Thanks, upon reflection that was about the same speed as your issue, for reference, the car in front ( entire 10min video) is Boom Pow with the boss himself driving, I was trying like hell to learn something...

Good catch, our car crests the same spot a little light in back but steer with it till landing, usually a giggle moment till the next turn where I reflect on how badly I suck at that hairpin turn.

Saying that

Is it better trying to Hero it out or just concede and safely spin?  Personal habit, from years of dirt circles filled with hillbillies trying to pit me every other lap, is to save.

Yup - I'd like to learn mucho more about how to handle 'off-nominal' situations like that, since otherwise I'm only going to learn piecemeal, the hard way.

Post up here if you find such a beast.

I'm of the mindset that if such an event goes on for more than a second or two the best solution for you AND the people around you is "Both Feet In".  When you start skidding under breaking your trajectory becomes a LOT more predictable to everyone else. As long as you continue to fight it you are putting everyone around you at much higher risk.

Lets face it, if you were that good you wouldn't be fighting it for any length of time anyway. You would catch it on the first or second wiggle and motor on.

If a situation ever gets to the point where you think to yourself "Hmmm.." Its already too late. Both feet in.

Here's how the handle it over in D-Spec land: https://youtu.be/FxuZ0EoTp10?t=2529

Make sure you throw in the victory yell at the end. :-)