A friend of mine on Facebook turned me on to this unique device. It was first demonstrated at the Musikmesse 2012 music dealer conference.
Martin designers understood this and worked with McGuinn to design an instrument with a normal G string and another string tuned an octave higher. This allowed Roger McGuinn to play his solos on a six string-like guitar, but get that extra high note on the G strings.
Addstring device. This allows anyone to attach a .009 G string to any guitar by means of a few easy to use attachments. These attachments are easily removable and will not permanently harm your guitar.
the Addstring, I will let you watch the video. One unique feature is the small bit that fits alongside the sound holes edge that allows the player do damp the high G string when not in use.
The cost of this device is € 59.90 plus VAT and shipping cost of € 6.50. I estimate this to be around $100 USD
If you can find a Martin HD-7, Roger McGuinn model, it will set you back around $4400 USD.