What's realized about these gains if the company is blocking the sale of the stock on the secondary market? This is toilet paper these people were duped into thinking was worth something. Sounds more like a lawsuit.
EDIT: That being said, I think it would be reasonable to contemplate regulations that prevent private companies from blocking secondary market sales if they offer stock options/RSUs to employees.
Let's hit the presses: tech startups hate engineers that think of edge cases
Either that, or IRS not consider the exercise as taxable until those conditions imposed by the company preventing secondary sale are lifted.