A question perhaps to help anybody else who finds themselves in this situation...

If one doesn't have the cash to prepay the taxes, where do you find a short-term lender for this?

Let's say your RSUs are worth $1M and you need to pay $220k in taxes in March, but you won't be able to sell the shares until a few months later.

In theory the $1M in public company stock seems like a fine collateral. But in practice, a recent IPO's stock can fluctuate drastically. Maybe they announce bad results in April, the stock goes down 50%, and you already paid taxes on $1M but now it's only worth $500k. An ordinary bank probably won't loan you the $220k with such a high risk collateral (I might be wrong).

If banks or rich friends are not an option, where do you get the $220k? Second mortgage on your home?