Anything is possible with a welder an angle grinder and a bunch of time, but the short of it is it will be a lot easier to put new bearings in a GL1100 comstar and sleeve the brake arm, then come up with a master cylinder arrangement. For a GL500 it's pretty easy. Weld a stop to bolt the brake arm to, weld a mount near the OEM brake arm, and use the arm pivot to actuate the master. A GL1100 comstar and disc brake conversion is probably the easiest one to do, hands down.
Charles.