Good luck with that considering most if not all of the other investments would be private investments that would be difficult, if not impossible to replicate in public markets.

Maybe you could find a bunch of other funds with slightly different positions in the same companies, and then form a linear combination that cancelled out the differences.

You'd end up with an "error portfolio" that was somewhat random that you'd be investing in proportionally with your SpaceX investment.

If I started a few listed companies and bought carefully constructed portfolios of unlisted securities specifically so that this would be possible, I wonder if the SEC would get mad.

I smell a SaaS product…

I smell financial regulation. The secondary market is generally only available to accredited investors. If you're investing in a fund, that's one thing, but if a product is unwinding all of that to allow unsophisticated investors to essentially buy secondary stock in private companies, the SEC is going to have something to say about that.