With the first caveat that I am an engineer, not a lawyer, I think there is a pathway to achieve some progress towards a X-Holding company.
The second caveat is that there are (e.g) ITAR-related restrictions on non-US nationals having shares in SpaceX because it is dual-use controlled technology, but I am unsure to what extent this is a real issue. Given that (e.g.) Lockheed, Boeing, UTC etc are publicly traded corps I don't think that is an absolute restriction. Instead I think it is that US majority control must be assured and that certain key positions must be filled by US nationals. So I think this can be managed.
The third caveat is that SpaceX is not a publicly listed company. (Is that the same for Neuralink and Boring ?). But bear with me as that really is a price determination issue and I think a way can be found.
So .........
1) Set up a company, let's call it X-Holding-Corp, and give it authority to list publicly and to issue shares.
2) Determine (i.e. hardwire into its Mem & Arts) that for each share it sells it will in turn buy one share of each of the following : Tesla, SpaceX, Neuralink, Boring.
3) Then, pre-IPO (or SPAC-route), X-Holding enters into an agreement with SpaceX, Neuralink, and Boring that they will issue shares to Holding at a defined price (i.e. one agreed to by the existing shareholders of these corps, just as if it were a new rights issue). Note that these would be for a common defined number of shares - say 100-million for starters.
4) Then carry out the first public offering for the 100m and the listing price will be the trading price of TSLA+the-agreed-prices for the other three.
5) Then after listing monitor the share price of X-Hold and if it trades higher than TSLA+ then there is appetite for more. At that stage either carry out new common issuings, or allow existing private holders (primarily Space-X) to swap across.
In a way this is no different than creating an ETF that has a defined and very restricted mandate - namely to only invest in Musk companies and with the ultimate goal of the ETF owning 100% of all of them. It must be possible because Barrons have an ETF(s) which mixes public & private holdings in TSLA and SpaceX in such a manner (BPTRX, etc).
Provided that the minority pure-play holders are not disadvantaged then it ought to be possible. The real advantage is that it gives SpaceX access to cashflow from Tesla earlier without going through the taxation implicit in issuing dividends and recycling those dividends in Musk's personal tax affairs. (Or capital gains if he goes down the sell TSLA, re-invest into SpaceX route).
So, I think it is possible. However, I do consider this to be unlikely.
