It would seem that this could be solved through negotiation. Pincus needs the shares now to recruit top talent, and without the shares everyone's chances of a great exit are diminished. Pincus could sell his own shares, but would risk losing control.
So Pincus should be willing to essentially write futures contracts on the proceeds from his own shares and trade those with employees, who should be willing to accept them in trade, even at precisely equal expected value.
The irony of course being that anyone good enough to be in his target market for "top talent" is probably going to take one look at a move like this and run away very fast.
Meanwhile, he's likely to see an exodus over the coming months of other good employees whom he didn't try to screw (this time) but who also now realise that the benefits of staying long term aren't necessarily worth the paper they're printed on.
You can pretty much write the rest of the story when something like this happens. The only question is whether people like Pincus himself will manage to cash out at some stage before Facebook intentionally or otherwise pulls the rug out from under Zygna in much the same way and the potential value of an IPO plummets.
So Pincus should be willing to essentially write futures contracts on the proceeds from his own shares and trade those with employees, who should be willing to accept them in trade, even at precisely equal expected value.