You could buy shares in GS or any other public company investing in private companies and/or underwriting IPOs, so there's that. That said, the idea that public capital markets are widely considered to be a way to increase equality, of all things, is stunning to me. I thought/think that people supporting capital markets view them as a method for companies to raise capital in exchange for a share of future profits, and that people opposing capital markets view them as casinos, pyramid schemes, etc., and that the point of extra regulation for public companies is to protect investors, while the point of having less-regulated private companies is to lower the cost of raising capital for those ventures that can make do with a small number of rich investors.

Even if people widely consider the stock market to be an "equalizer", it doesn't seem obvious that adding more early-stage small-cap stocks to the market serves that purpose, because these companies are more likely to go bust, with a small fraction of them capturing most of the growth, and an index with a share in all those companies is not unlikely to underperform a large-cap index. (Incidentally, this is related to the reason that widespread stock ownership seems to me more likely to increase rather than decrease inequality - the prices of these things have a high variance; taxing and redistributing wealth and/or capital gains sounds like a surer path to equality, if that's what you want.)