When I was at the International Pedal Steel Guitar show in St. Louis a few years ago, the Paul Beard Co. had a 7 string Reso on display. It was setup with the 6th or b7 in the middle of the tuning. This seemed to work very well. I've wanted one since. I believe the tuning
was G B D E (or F) G B D. With a six sting there are still lots of possibile tunings. In a Open D tuning (D A D F# A D), tune the first string down by 1/2 tones to get a DMaj7, D7, D6. The DMaj7 has a sweet Jazz or Hawaiian sound. Dave