I see.....

What happens if the download includes a data table or data pointers with the unused opcode as a totally legit value in the data?

Why don't you consider what is often done and write a monitor download that simply accepts Intel Hex file records. This way you get a framing protocol, an address field, and a check code to validate the data. And to download you simply have to send the HEX file to the target.

You may even be able to get Free code for this if you ask here on the forum.

Michael Karas