When you use the rear brake pedal, you get the rear brake and right front brake caliper exclusively. The front brake lever only activates the left front brake caliper. Changing the activation lever for the rear brakes will not change the use of the system. You mention that you predominantly use the rear brake and not the front; however, you are using one of the front brake calipers as well.
Another issue to consider is what would happen should you get into an emergency braking situation. Most of us drive our cars/trucks more than our bikes. When an emergency situation occurs, most of us go into auto pilot and revert to what we know most and best, that is how we drive our cars/trucks and the first are we go for is the rear. If you change the activation of the rear brakes to the handle bar brake lever, you will loose a lot of braking power further compounding an already stressful situation.
The procedure that I have been taught for emergency braking on all the advanced riding courses is rear brake, front brake and downshift, in this order. Unless you practice emergency braking on a regular basis such that you have the procedure engrained into your muscle memory, the front brake is the last item considered in an emergency situation, but one of the most important.
Changing the rear brake to activate only the front brake will only give you the left front brake caliper for stopping power instead of the rear brake and front right brake caliper.
Having stated the above, the last issue to consider is what your insurance company would do if you were to make such a significant change to an essential safety system on your bike. I truly like innovation and will try most anything to mod an older bike, but will not play with the brake system.
You must also remember that these bikes were not designed to use sidecars, or haul trailers around. These are all aftermarket innovations that we, the riders feel are great assets. I would recommend you change your brake pads yearly (pads are cheap), and overhaul the calipers every two years.
I would go further and recommend that you have your clutch rebuilt as well as it will undergo increased stress and wear more quickly than if you were not using a sidecar. I have had to pull the engine on my '85 LTD to do stator work and on recommendation, have rebuilt the clutch. The PO towed a trailer and there are 120,000 Kms on it. The steel discs were overheated and scored, and the damper plate rivets are extremely loose. The new damper plate that I will install (can't bin the damper as there is too much play in the clutch stack even with the addition of two additional steel discs) has no movement between the steel plates.
Just a few thoughts on your issue. Good luck.