I started a thread where I was trying to sort this issue out myself. I thought you might find it interest and I would welcome your thoughts.
Account Audio/Subtitle Settings: How to get Plex to choose original audio?
Your approach here is quite different then what I was hypothesizing. In any case, I am happy to vote for your suggestion, as it is my single biggest annoyance with PLEX. I have also proposed a much simpler solution here that might interest you as well:
Give Option to Play Default or First Audio Track
cheers,
-a
