If you're going to use mic cable anyway and it's longer than 3m, consider this one:
http://www.tubefreak.com/buffer.htm
... the way I built one had a small (50x50x25mm) Hammond cast aluminium box to plug in the 9V, very close to the amp side jack. Nice site BTW. I have to apologise for the English; I offered the guy to iron out the wrinkles, translated some pages but he never put them online.
Ok it's not exactly what you asked for, but it relates to the topic ?
Now you say you want to use the shield as a ground on the mic cable and hook it up on one end only, using the second core as the real ground; I would be worried that the capacitance to ground then is higher than with the ground on the shield at both ends and the second core left unconnected. Not a problem on active guitars or Tele's, but loss of sparkle on the rest I suppose.
