But you also can't close over anything right?

I did a spiritually similar thing in JS and Dart before where we read the text of the function and re-parsed (or used mirrors in Dart) to ensure that it doesn't access any external values.

You actually CAN close over RPC stubs. The library will capture any RPC calls made during the mapper callback, even on stubs other than the input. Those stubs are then sent along to the server with the replay instructions.