After some trial and error I finally devised a solution.
I first tried CRC disc brake quiet. A reddish orange silicon type compound applied to the backing plate of each pad. It killed the squeal but only lasted 50 to 100 miles. Examination revealed that it was totally resistent to the heat but the pressure of each piston would squeeze it out and the squeal returned once the layer between the pad and piston became too thin.
Next I tried some black gorilla tape I had laying around the shop applied to the backing plate after reading some posts about people using tapes like duct tape, etc. It too completely eliminated the squeal; however, as expected after reading several expert opinions it was obvious that the tape's adhesive simply could not withstand the heat. I was concerned about the goo getting baked on to the caliper pistons so I removed it after observing that the outer edges were curling up.
Impressed with the heat resistance of the CRC and effectivness of the tape I decided to use the heat resistance of the CRC to insulate the adhesive of the tape from high heat.
I cleaned the backing plate of the pads reapplied a coat of the CRC disc quiet compound and baked the pads in a 250 deg oven for about 5 minutes after first letting the compound set at room temp for 10 min. I then applied the tape to the back of each disk pad and neatly trimmed the edges with a razor blade.
Some mileage and time will tell, but so far the tape has remained firmly in place and I haven't heard a peep from the brakes. It will be interesting to see if this combination will last the life of the pads. Seems that it is just as good a jury rig as taking it in for warranty and having HD service use the "paste" specified in the service bulletin. Besides, if your pad thickness is below 4mm like mine (3.73mm after 670 miles) Harley won't cover it anyway.
I don't intend on replacing the pads with HD pads because I don't trust the quality after this fiasco. Instead I will try Lyndall Z-plus pad on the next brake job. Apparently the Lyndall Gold racing pad is too hard for street use and some folks that have installed these have reported great braking with some squeal. Some have not.
Brake squeal is so irritating I don't plan on taking any more chances with metallic pads.