Yes, in principle, any sort of remote compute we want to support, we could accomplish by having a custom function you have to call for it. Then the calls can be captured into the record.

But also, apps can already do this themselves. Since the record/replay mechanism already intercepts any RPC calls, the server can simply provide a library of operations as part of its RPC API. And now the mapper callback can take advantage of those.

I think this is the approach I prefer: leave it up to servers to provide these ops if they want to. Don't extend the protocol with a built-in library of ops.