I have often thought that it would make sense for employees of different companies to pool any equity. You already take on enough risk being employed by a startup business without adding on your equity risk too.
More seriously there is a number of ways of doing this. I would imagine the simplest would be to create a trust and have everyone give their equity and options to the trust.
The bigger issue is how to ensure that people don't game the system. The assets being put into the pool need to be accurately valued so people don't put in what they know are worthless assets. Of course there is there is the reverse situation where those already in the pool undervalue new assets.
Most of the effort of this idea needs to go into setting up the rules so that everyone does the right things and everyones interests are aligned. While I think this might be hard, I don't see anything that can't be solved by some smart people.
Don't get me wrong, it seems like an interesting idea. I would love to have some way for my un-diversified work portfolio to get diversified. But short of creating a sweat equity only market/exchange I don't really know how you'd do it.
Even that has problems because people could fake a startup well enough to gain access to the returns creating a free-rider problem. It's probably easier to cook up a bunch of buzz and interest over an orchestrated 3 month campaign (esp. with a Kickstarter) than to actually do a startup. And since you're faking it you can promise the world and raise huge money from unsuspecting dupes with no intention of (much less a plausible method for) making good on your promises.
Now multiply this across dozens of companies. Plus, most startups give options, not equity unless you are a very early employee.