Comma operator I would agree is really only used for shenanigans. But operator= is as crucial as the copy/move constructors for ownership, so I'm not sure how you picture this...
> apparently in C++ a structure cannot really have zero length
Yes it can: https://en.cppreference.com/w/cpp/language/attributes/no_uni...
> I think it should be possible for your own structure or union type to overload any operators that are not already defined
I see that you are trying to solve the problem of "when reading this code, I don't know if these are the built-in operators or custom ones". I would be interested in learning about situations in which this was a problem for you, because I have not made that experience myself. From my perspective, you need to know the types of x and y to know in the first place which operations are possible - default or custom - so this rule does not really buy you much. But again, I don't think I understand your problem well enough.
FWIW, you can define a custom operator-> and operator* (they have to return pointers, or types that themselves have operator->/operator* which are then applied recursively). I am not convinced that mixing pointer-related semantics into e.g. object assignment is a sufficiently elegant solution from a conceptual perspective though.