Not surprised to hear the idea's been around a while. It's too simple a concept not to have been done before.

One old bluesman noted for using a guitar with added strings was Big Joe Williams, composer of the blues classic "Baby Please Don't Go". Pictures of his nine-string guitar HERE.

BTW, the salesperson at the store where I tried out the HD-7 said that McGuinn had the guitar built because his aging hands were having trouble handling the larger neck of a 12-string. He wanted something that was easier on the hands while keeping some of his signature sound.