one option is an artificial ground, essentially another tuner placing a counterpoisehttp://www.mfjenterprises.com/Product.php?productid=MFJ-931
you could run a nice long #6 wire down the outside to a ground field, too. probably need some ferrites on it to kill spurious resonances. the artificial ground does not provide a DC ground, and a long ground wire introduces RCL components of its own but is going to be more electrical-code friendly.
the old 1920s "hook a wire to the cold water pipe" trick has drawbacks in these days of plastic pipe, and it is alleged it could make the pipes "bite" if you touched them key-down, too. not the best solution.
in any event, putting a real solid groud that high up makes you a little more inviting to thunderstorms...