As a practical matter, what I've seen in SV companies, is that there are networks from folks that new each other in the service. Once one gets a role in a company, many other skilled veterans are sure to follow. The same thing happens to group from other prior companies that get hired in a waves, often following a particular senior leader brought in for a new initiative, or a new startup. I guess this doesn't scale to veterans as a whole, but it does show how even a little support can go a long way.

This applies to many excluded groups. People hire from networks; if you don't make any effort, your network will be the status quo, which last from generation to generation. With a little effort, you can expand your network to excluded groups. From even a purely self-interested perspective, access to more talent and a more diverse set of ideas and experiences is never a bad thing, and being exluded means there is more undiscovered talent.

One simple method is to tell the recruiter that you want at least N serious candidates from excluded groups for each job. This causes the recruiter to also expand their network.