I made a mini example in C.
It's awkward to do get right because you need an indirect pointer whose address remains fixed, but points to another pointer which can change (and is volatile).
While it might be possible to make something like this lockless - it's much simpler to stick a mutex in the array header. When we access the array_segment we can take a lock to prevent some other thread reallocating mid-way through accessing.
There's probably a few improvements that could be made. In particular it doesn't handle use-after-free, so it's not thread safe w.r.t cleanup.