The November 2007 issue of Lost Treasure magazine had an artical about the Gould-Bazooka that had been converted to dry washer system. Gould Engineering has a web site but I could not find any info about the dry washer.
The spinning vortex in the long tube would certainly separate heavy from light, but I agree the efficiency might be low. But, if you put your concentrates in again..you might be moving alot of material through....the name of the game is to end up with the gold!