This is maybe rare, but there's at least 1 late-stage startup that gives RSUs and allows you to do this in practice. Where my friend works, they give you the option to either pay your own taxes when your RSUs vest, or they will purchase an amount of RSUs from you to cover your tax bill. Seems like a really good deal for the employees. I don't know if this can translate to options in any way, I'm only familiar with RSUs.