This DID however get me to thinking how relatively easy it should be to add a valve similar to this but set it up to drop the horn a fourth like a pic... now THAT could be useful on a Bb or C. :)[/QUOTE]
This has already been done many years ago. Check the ITG archive journal articles and pics. Always thought it would be a wonderful horn to play.
The Courtois 4-valve flugel is this type of set up as well.