Here is a well known piece for tuba that features multiphonics and beat-boxing. Fnugg (Øystein Baadsvik), as performed by Chris Dunn at the BBC Young Musician of the Year Brass Final 2012.

https://www.youtube.com/watch?v=fGVxt8BYakM

I was lucky enough to see this performed live by Sasha Koushk-Jalali at a London Contemporary Orchestra event last year.

Thoughts?