I had a smaller YC company pitch me something like this as an option for my stock comp - an RSA (restricted stock agreement, or "founder's stock"), where I put up all the cash up front, paid a big income tax bill in the first year, but then upside was all capital gains. I would technically own the stock but I had to sell it back for nothing if I left before it vested.

Turned out I left very early because the company wasn't doing great, in the current climate I think they're probably default-dead. All that cash is just gone.