Ask HN: Secondary markets for employees with stock options?

Many great minds have discussed how the current employee equity situation is broken. One point is how the 90-day stock option exercise period forces employees to give up vested stock if they are fired/leave, if they can't afford to pay the exercise price of their options and the Alternative Minimum Tax (28% federal of "paper gains"). Some have responded by changing their exercise periods to 10 years e.g. Quora.

For employees who lack that option, what about secondary markets that offer share liquidity before IPO/acquisition? Employees can exercise all shares and sell enough to cover AMT. The first wave (SharesPost, SecondMarket) offered direct transfer of shares on the cap table. The next wave (ESOFund, 137 Ventures) can do deals without company involvement, with a non-recourse loan with limited upside/downside, or a forward contract with cash delivered today, and the certificate held as collateral until IPO, when it is transferred.

So why don't see this discussed more?

One reason is some companies try to restrict anything involving shares-from Right of First Refusal, to an outright prohibition of any loan/sale/_ without board approval. Some argue the latter is unenforceable, but I haven't seen any case law. First-hand knowledge online is scarce and lawyers give unclear answers due to the novelty of these deals. Can the company find out? Intervene? Sue? Are they likely to? Would this cause an unseemly public case and TechCrunch headline? How different is a rich relative vs. angel vs. a marketplace investor?

I'm asking HN to get more thoughts on secondary markets, and new ideas on solving the stock option issue for employees.

Background: https://amplitude.com/blog/2015/12/01/employee-equity-is-broken-heres-our-fix https://zachholman.com/posts/the-new-10-year-vesting-schedule https://dangelo.quora.com/10-Year-Exercise-Periods-Make-Sense https://news.ycombinator.com/item?id=7920877 http://blog.samaltman.com/employee-equity