Who says your fronts are wearing more than the rears? Maybe not the case. Only way to do the experiment correctly would be to start with 6 new sets of shoes, measure and install them, drive 40,000 miles or whatever, then remove and measure. Then, do the experiment 3 times total.
In this case, I would guess all 6 to wear the same due to the common pressure and rotational RPM. As others have mentioned, carmakers take advantage of weight transfer and use much more powerful disc up front, and lower cost drum in the rear, until recently. This is the point that is getting mixed into here, which is not the case in this situation. Now, I believe the majority of vehicles utilize rear disc also. Another removed maintenance point as they are self cleaning and self adjusting. Obviously a disc brake system is much more costly to manufacture than drum, due to higher operating pressures, much larger components, and the machining of 2 friction surfaces (rotor) vs 1 (drum). I converted my Jeep Cherokee over to rear disc (Jeep Liberty setup), mostly due to the fact that I continuously launch trailer boats/watercraft, and the drums would get wet, and stay wet, adjusters would seize, etc.... I also now enjoy a very firm brake pedal with only 1/2" of free travel before the binders are on and the pedal is a rock. I hate drum brakes, they belong on Model T's, and nothing after.......personal opinion.
Just in case anybody is confused, a proportioning valve limits or reduces the pressure to the rear brakes, so they do not lock up during panic breaking. It also contains a shuttle valve which will move and indicate that one part of the system developed a large leak. The wire connector on this valve is a detection switch going to the dash "BRAKE" light, which should also come on if the parking brake is on, or is the reservoir is low on fluid (newer vehicles). The front disc system is allowed full uninterrupted pressure directly from the master cylinder.
ABS is to maintain steering during panic stops. Though the coefficient of static friction (rolling tire) is higher than the coefficient of dynamic friction (skidding tire), the primary function of ABS is not to decrease stopping distance, as some believe. On recent vehicles, the ABS distribution block handles the proportioning of the rear brakes, and therefore a separate proportioning valve is typically not found.
Above paragraphs are totally off target, but, I'd place my money that all 6 would wear about evenly in this thread situation.
-Soldier B