I just came across this on the TromboneChat forums. Might be worth a look by some of us since it shows a number of instances where people are using these supports with euphs and tubas. I don't think that I'd make use of one since I use lap supports/stands for the euph and tuba, but if I had to stand up and play, I'd probably think seriously about using a hand support. I'm mulling over trying one of these supports (similar ones are available from other sources as well, mentioned in the original posting) for my bass trombone, where I'm currently using a "bullet brace".

Trombone grips