Brian,

Soon - USNWC staff have tried it and liked it. Keep asking USNWC about
riverboarding (short answer).
Last Fall - I was able to convince the paddlesports director Dave Hepp
to try riverboarding and he along with several others including the
Rafting Manager, Aaron had a blast riverboarding USNWC. Dave told me
they are working to get their planned activities up and running smooth
and then look to add riverboarding.
Give USNWC a call and ask to speak with Dave - with some more interest
expressed in riverboarding - they will speed up the approval process.

Shane