Those RSUs are a particular kind of promise, and it makes sense to use RSUs when the company has gotten very valuable because you hit the ISO cap pretty quickly

Those RSUs are a particular kind of promise, and it makes sense to use RSUs when the company has gotten very valuable because you hit the ISO cap pretty quickly and NSOs already have a tax-at-vest issue. My guess (IANAL) is that you'd just change the promise. "You will vest these RSUs when we go public and you can pay the taxes". I actually think there's _already_ an issue around this, because you can be vesting RSUs while the company is public but you are personally locked out and not allowed to trade them -- and yet you need to pay your taxes and the normal way to do that is to sell a portion of your RSUs. In the case I know of (Twitter), there was a special way of handling that on a one-time basis. I don't remember the exact details (it didn't apply to me), but they managed to work it out.