Digeridoo!!

This is an awesome idea.

You are completely correct in that the tube
doesn't matter so much as the vocal tract
upstream.  The tube could just be a delay-
line (like Karplus-Strong), but the lip model
and the resonances back of it are where the
real action take place.  

Check out the Brass UG in ChucK (and/or STK),
you could possibly just attach some simple 
resonant (formant) filters to the lip, and get
lots of dig-like articulation by moving them
around.

Forge Ahead!!