There is no proposed mechanic here - until the origin story bakes a bit more, I would like to avoid adding first class indirection. That said, I would hope that this would just be a “heap box” sort of thing, rather than a feature of enums. “Simple things that compose” are preferred over dedicated features IMO.
Sure, I can see a use-case for “explicit layout control”. If/when structs get that, it would be natural for enums to have it as well (since enums would be sugar for structs). I don’t see a reason to do enums first.
While appealing, I’m not aware of how to make this general purpose. The problem is that you want an instance of an enum to have a reliable in place memory representation - this is required to get references to it.