Publicly traded companies with exposure to private companies:


The insurance conglomerate Berkshire Hathaway BRK buys 'private' companies.


There are closed end funds with large positions in private companies, MVC EQS. There are also open end mutual funds that invest in private companies, but it seems insane for an open end mutual to so that (due to redemptions). EQS is the closest thing to what you described.


There are also finance companies PMC, PCC, DOLL ... that finance small businesses.


I own BRK PMC and DOLL.