Easier said than done. You can already have:

    unsafe fn new_in_place(out: &mut MaybeUninit<MyType>)

But you require unsafe code both to implement it (to project MaybeUninit) and to call it.

For it to be safe to call the type system would have to encode the invariant that &out is initialised after the call. It becomes even more complicated if the operation can fail.