Yeah, I am also aware of 7-string guitars with a truly "extra" string four of five half-steps above the high E or below the low E. I think an extra bass string is more common than an extra-high treble string.

The great New Orleans contemporary jazz player Steve Masakowski (sp?) plays a custom-made 7-stringer made by the same luthier who I patronized for a refretting job last year. If you try to Google a reference to Steve and can't do it because I spelled his name wrong, try the name of his primary band, Astral Project. (Steve's extra string is below the low E.)

Back to the idea of a six-stringer with one additional "paired" string in the G-string position, I think there was a Mudcat thread a few months back about a new "signature" Martin model in that configuration, commissioned by Roger (nee Jim) McGuinn. McGuinn, of course, is well-known for his electric 12-string sound as founder/leader of the Byrds; he apparently uses a 7-string acoustic these days to create a sense of that "jangly" sound without doubling up every string.