It is `*mut UnsafePinned<[u8]>` sound for shared mutating memory (in process or inter process)?

Sorry about that/
I might got into some x-y problem and oversimplified the example as I ripped it from a larger private project.
The core part of the problem is, can we implement a address-free SeqLock soundly based on AtomicPerByte RFC? If we do, how to represent data on reader and writer?
Here is the more concrete question: Is this`SeqLock` based on atomic memcpy sound?