A little known -- and very fine -- guitar maker named Don Rust in Ursa, Illinois (10 miles north of Quincy) made the only dobjo I have ever seen. It IS quite a critter at that.
However the concept really makes quite a bit of sense since standard "bluegrass" Dobro tuning (open G) is basically banjo tuning without the drone string, and a lot of Dobro players are recycled banjo players.
While I'm plugging Don, I'll say that he has made flattops for both John Hartford and Greg Brown, and all his guitars are really something special. Good prices, too, and some of the most select woods to be found anywhere.
Best wishes, TONY