Pack raft will work.
Dr. Duane Vandenbusche
Ran the black canyon before modern day kayakers (he did two five day trips in the 1970s) I believe in a pack type raft.
He also wrote the latest history book on the place. See link below.
And yes, the first successfully decent was done on an air mattress to find a place to dig the Gunnison Tunnel.
The Black Canyon of the Gunnison - Duane Vandenbusche - Google Books
That being said, I believe that a kayak is still the safest fasts way to make the decent. It is truly a place of class V/UR eddy catching. Miss a must make eddy and you will likely pay with your life.
However, Dr. Vandenbusche, has said, if he ever goes down again it will be on an air mattress.