I have converted my 1998 fsr ground control comp using an A2Z disk brake adapter and a couple of rose joints and a turned/ tapped length of aluminium to secure it to the brake boss. It works a treat.
I have also used a kit from rock'n'roll bikes called the onza disc brake adapter/converter I think and I converted my sister's Zaskar LE using it. About thrity quid for the kit I think although an unused one just went on fleabay for £18. This comes with a longer QR skewer to accommodate the extra material butting onto the dropout.
Regards