[@Aleksey] Very good point! I'd missed that vector does the SOA->AOS business too. (I thought that was just Repa & Accelerate.)

In any case, an unboxed vector of tuples can't actually support atomic CAS anyway, so not supporting it is inevitable.

It does mean that exposeMutableByteArray can't be a method of the Unbox class. It would need to be another class capturing the subset of vector types that are "Atomic" (support concurrent/atomic memory ops).