This is basically how defunctionalization is implemented. I'm less confident about the "what if the destination doesn't know about the msgType you sent" issue,

This is basically how defunctionalization is implemented. I'm less confident about the "what if the destination doesn't know about the msgType you sent" issue, because defunctionalization itself is a whole-program technique where the destination context has to know what to do with your function call. But your solution (i.e. raising an error condition) looks reasonable enough.