Equity is compensation. You can't tell your employees what to spend paychecks on, and you shouldn't be trying to block them from gaining liquidity. Maybe a doctor just told them they have a year to live, and won't be around for your planned 2019 IPO.
Unless you are granting them voting shares it is unfair to lock them into your vision of the company going forward, especially if they have departed.
Voting is a non-issue. Assuming things are structured right, the founders should retain majority control. The investment is primarily an investment in their vision, not something to be micro-managed by shareholders.
"I am Paul Buchheit, I made Gmail" seems like it would work for you, and something similarly awesome might work for like 10s or 100s of founders.
As a Google employee (with pretty high, low variance, liquid compensation), it seems having stock/options or options even a successful pre IPO company are kind of terrible. When you seemed to have won (worked for a successful company, vested), you don't even really win, or at least, not yet. Stay in that same desk, working on that same project until some indefinite future liquidity event happens.
My point was exactly about vision. Founders vision often change from what was pitched when you join.
Let them have a say or let them out at a fair market price.
There's also no reason why equity must be illiquid -- or skewed to benefit a company more than the employees that built it. There are ways for people who received equity as compensation to take a little out on terms that are fair for everyone. Just because it's always been done a certain way is not a good justification for this position... after all, Uber is breaking the taxi business in a similar fashion.