They could not, because he does not own those businesses outright. Doing so would violate the other owners rights. They can seize his shares in SpaceX, but not SpaceX property.
But the end result is always the same. You can either voluntarily liquidate and pay your judgements or you can have someone else do it for you.
I have not done an analysis on exactly which of Musk's private companies are jointly owned, and how the ownership is structured. It's a comment on hn for crying out loud.
The only question is regarding which entity is liable. If a judgement were entered against SpaceX, for example, then absolutely rockets and real property owned by SpaceX can be directly seized.
Without looking, I'm certain there are at least a few companies owned by Musk which could have their assets taken directly.