The most inspiring use of the 7 string (the added bass string kind, not a doubled or octave g)that I know of is in the Brazilian Choro bands from the turn of the last century, kept popular through the 1950's by Jacob Bittencourt ("Jacob do Bandolim") and some of his contemporaries. Check Grisman's Acoustic Disc catalog for Jacob's music. The characteristic sound in some bands included- egad- saxaphones, but the first volume of Grisman's production of Jacob's recorded output is defined by the interaction of Jacob's lead mandolin and the seven string guitar. There is some jazz use still- Pizzarelli was mentioned in an earlier post- and anyone interested might want to find some George van Eps. Some of his canon is hard to find, but eminently worth the search. I know at least one serious blues singer putting an Eastman seven string to good use, T J Wheeler of Portsmouth, NH.
Carter Bannerman