I finally got a chance to apply the patch to Main_MiSTer and try this out... looks pretty good ! (for cdrdao -> CHD rips)
However, I was wondering whether you were expecting it to work with CloneCD-type rips... it doesn't. Or rather, it plays them as audio CDs only.
The changes to Main_MiSTer would seem to imply that it should be opening the *.sub file, and reading from it though...