This just isn't feasible for virtually all employers (minus a few startups and the rare organization that isn't risk adverse) for a variety of reasons. They're losing an employee 20% of the time, they're getting a cut of something they have no interest in and don't really understand, and they'd have to spend resources to understand the contract, legality, and valuation of your venture.
In other words, for the average organization there is virtually no upside.
The only caveat to this is if you do a true intrapreneurship type deal where you are building something within the scope of your employer that they might leverage for operations or as a product sometime in the future. But that would most likely require that they have a clear first right to purchase or that the ownership roles are reversed and you are the minority holder which isn't very ideal for the 30-something.