You can offset charitable donations against income tax. The personal capital gains is avoided by transferring shares directly, without selling. (The charity typically then pays the capital gains via unrelated business income tax.)
> Alternatively, have you tried selling part of your stake to the new investors in the round, at a discount?
Unfortunately, no. I tried :(
Just a thought: having a large carry forward tax loss in itself has value, and if it were a sure thing you would get the deduction (e.g. through an IRS revenue ruling), then if I were you I'd probably look for some way to team up with someone. Ethics aside, there are probably ways to help someone else avoid taxes using this loss. Maybe you can make a million dollars, by helping someone else save 5 or 10 million in taxes.
Honestly, not a bad idea, but perhaps worth getting an IRS tax ruling for, since we're talking tens of millions (paper gains). However, you'd be set for life then, and the IRS wouldn't be able to challenge that decision in the future.
Alternatively, have you tried selling part of your stake to the new investors in the round, at a discount? If you could sell 10% of your stake, discounted, you just figured out the entire (actual) value of your equity stake.