Bes-ama, Yama-son, uhh that is...

[iiipopes]

The thing is, will the valve block fit in the bows? That might take experimentation. I thought about the same thing to get a comp block into a sousaphone for indoor gigs, and quickly gave it up for that reason. I just had the top loop of the 1st valve tubing of my sousaphone made into a valve slide, since it sits perfectly under my left hand. A lot more usable, a lot less grief.