Oh, ok. I thought this would be a great option to have. I saw a piece of code that prevents a nsis installer having two of the same process running. Something like CreateMutex or similar, not too sure.
And yes, spur of the moment it was, I completely forgot there were other modes to worry about, but you could send a custom message saying "change to write mode", then send a drag'n'drop message with the double-clicked iso as the data.
Just tossing a possible solution around here. If its too hard, don't do it.