My first thought is that you don't get something extra for nothing, and the payment for this new feature might be a reduction in the odds during normal play. The other problem is that a player has to commit their bankroll to a single machine, and some may want to move to a different machine if they are having a bad run.
A negative credit meter might have some players thinking they will end up owing money to the casino, and this may be putting them off. Redesigning how the meters work should eliminate this fear without changing the underlying concept.
I have seen similar ideas online. Lasseters have a satisfaction guaranteed bonus offer for regular players, (if they have not removed it), that measn that if players lose everything within the first hour, they get their bonus credited again for another chance (with a new
WR of course).
A few MG casinos have tried not offering a sign up bonus, but a money back guarantee for a deposit of £50. If you lose it all, they will give it back to you in the form of a refund to your deposit mechanism. Sadly, it doesn't always work. I tried one, lost my £50 and the system said I didn't qualify even though I had zero balance.
One possible way to implemement this online is to say that players can deposit without an SUB, but should they lose they get a a refund as a bonus, and this continues until they are able to cash out, say, half their deposit, or until a certain amount of time has passed, during which they can be topped up as often as they like. In this way, players will be guaranteed a period of play, and at least to be able to cash out half their deposit at the end. The casino will make half the players deposit as profit, except for those that win and cash out, which will automatically end the guarantee.
There would need to be game restrictions, probably slots only, otherwise players could repeatedly double up on Blackjack or Roulette and pretty much guarantee winning twice in a row or more and the casino would always lose. Bet limits might also have to be reduced as in the free play promotions.