John,
Having totally failed to get anywhere with your more complex suggestions, I went back to your first idea of pasting Pronto hex codes directly into the MX-850 Editor (I have the latest version). I found a selection of Sony DVR discrete power codes in the Pronto section of this website and pasted them into the Hex Editor that sits inside the MX-850 Universal Browser. I then used the drag and drop facility it provides to dump them on LCD buttons. After a few failures, I found a pair of codes that worked.

Thanks for pointing me in the right direction. I got there eventually!

David