I met a girl, many years ago, who also played a 7 string guitar (professionally). Her name was Jill, or Gill, I think but I can't remember her other name. She had it specially built (well, which shop can you BUY one in?) and used it for traditional tunes.

I can't remember exactly how it was tuned, but I think it was as standard but with an extra treble string tuned higher than E