I’ve been staying out of this discussion as I usually just hang in the 99 and up forums, but I’ll throw in my $0.02 worth. And I'm going to get wordy while I'm doing something else.
I actually liked the mechanical prop valve located at the rear axle. When working and properly set up, it proportionally kept the rear brakes from skidding too early and provides reasonable full braking effectiveness. These were not only used on pickups, but Chrysler Minivans also used them. There are some issues though.
One is of course that it is mechanically working. The second is that it is setup correctly, which in order to service you need pressure readings from both the front and rear brakes, usually lacking in home projects and often lacked even at dealerships.
Another issue is that it’s not ideal. Disc brakes are very linear in braking. How much pressure is applied gets you corresponding braking. Duo-servo drum brakes are not. The output increases based on both the line pressure and the servo effects. So if you’re applying 2000 psi rather then 1000 psi, your going to get a whole lot more rear braking then expected.
Proportioning valves don’t account for this; even the older props located by the master cylinder or the adjustable ones. The older set prop may have been designated 300/.43. This means you have full pressure to the rear brakes until the 300 psi point, then the rear pressure is proportioned at a 43% of everything over 300 psi. In a hard stop at 1000 psi, you’re getting 601 psi with that prop.
What ends up in the engineering lab is that you make a compromise in the rear braking ability. You might design the system so the rear brakes will not skid worst case with no load in the bed, but then your not getting full braking under heavy load (weight preventing tire skid) that you could be getting. All of this depends on your expectations of how hard the driver will end up pressing on the pedal. If the truck is lightweight and he is making a close to panic stop, will he restrain how much he is pushing on the brake to the point of incipient skid and getting ideal stopping distance. Or is he going to be heavy footed and just skid.
The RABS was an attempt to correct for the both the set point prop issues as well as the mechanical issues of the height sensing prop valve. Adjust the pressure only when the deceleration rate is theoretically high enough that a skid would occur. Basically the software reads how fast the tone ring in the differential is slowing.
But I hate RABS. Drum brakes by their nature have a high degree of
hysteresis– they don’t react fast to apply and release as disc brakes do. And neither does the RABS valve. Plus the RABS system does not incorporate a pump to regenerate the hydraulic pressure that it dumps. So each dump and re-engagement of the rear brake circuit requires volume from the master cylinder, so you end up with constantly dropping pedal while it’s doing its thing. The 4 wheel ABS just does such a better job. But it does end up providing a better stopping distance for those who are not that informed or trained about applying the brakes to generate incipient skid. That is if the prop valve is first selected properly from the factory.
To the question can you set up a proportioning valve in place of the existing unit? Yep, at your own liability risk. The common adjustable valves have been around for a long time, originally supplied by Kelsey-Hayes, which is now TRW. They are a simple valve, not like the combination valves of years past, where those valves included a pressure differential valve to limit fluid loss with a system failure, possibly a metering valve to slow application of the front brakes (allowing the rear brakes some time due to their hysteresis), and an electrical switch to light up a dash lamp to tell the driver of a problem.
The typical 2-port adj prop you’re going to find will have a set slope, like the .43 in the example above. What your able to adjust is the cut-in point, from 100 psi to whatever. They are not hard to install and we used to do this all the time when developing new friction material formulas for a new vehicle platform years ago before 4WABS. The new line is run from the rear brake port of the master cylinder into the cabin, then a return line back out to the existing rar brake line that used to be attached to the M/C. Connecting this up can be an issue, but NAPA stores carry WeatherHead fittings that allow you to do this. The guy behind the counter will not be able to match up parts, so it will be you looking through all the bins in the display to find what you need. The other thing you need to do is disconnect the existing prop valve and bypass it with hard line.
Here are some reference materials.
Proportioning Valves
StopTech : Balanced Brake Upgrades
www.tiltonracing.com/pdfs/98-1261_Prop_valves.pdf
Now keep in mind that your taking on the liability for how well your vehicle stops. Some S-O-B like me 
may be called in if you have an accident and run into someone, and the police, plaintiff, or insurance company sees what you have. And the acting accident reconstructionist will then evaluate if you did a good job of engineering your brake system, and if it had been properly adjusted the vehicle could have stopped 20 ft shorter and my client would not have been injured.
For your last question, there was no "load sensor". The mechanical lever connection adjusted the prop valve depending on the sag of the vehicle (bow of the leaf springs) between the rear axle and vehicle frame.
