I would also be really interested in a dataset like this. Institutional investors for public companies are sometimes listed in the SEC 10k form, but I have yet to fully learn the structure of these documents. Someone intelligent could probably make a web crawler to scrape institutional and single person ownership of companies from SEC filings. Ideally with a proper dataset like this you could map out who owns each company, and who owns each of the owning companies etc. which would be really interesting information.