TubeNet

I have had Amati style water keys added to a few of my tubas to try solving that problem. They work OK, but you have to keep the internal bits lubricated or they can stick in the open position. I use valve oil, but there is probably something better. They are small and can be put just about anywhere.
http://bassic-sax.ca/blog/wp-content/up ... -valve.jpg (not mine.)

I once asked David Fedderly if there were any negative effects of adding a water key, and his reply was "yes, one...it costs you a few extra dollars."

Go for it!