I am not an expert on proportioning valves by any stretch of the imagination, but here's what I DO know:
The "proportioning valve" used on zillions of GM cars is actually 2 things in one. Obviously, it's a proprtioning valve to reduce pressure to the rear brakes when you dig deep into the pedal, preventing the rears from locking up prematurely as weight transfers off of them to the front of the car. That part I think we can all understand pretty easy. (Although, I am as clueless as the next guy as to how this little miracle is achieved. In all honestly, if anyone's got a link showing how it's done I'd love to learn!)
But to the point at hand....
The OTHER thing that little chunk of metal does is act as a Pressure Differential Sensor. A pressure differential sensor is nothing more than something that measures pressure from both of the lines coming into it from the master cylinder. If everything is working correctly they should be the same pressure ON THE INPUT SIDE. If they are different it moves a piston that can contact an electrical trigger on either side, which grounds the "brake" idiot light circuit and lights the light on your dash. It can move in EITHER direction- front pressure higher than rear lights the light, rear pressure higher than front also lights the light. Like a piston with brake fluid chambers on either side of it. If one chamber is higher pressure than the other it moves the piston over, makes an electrical contact and lights the "brake" light on the dash. Please note this is on the INPUT side of the valve- WHAT PRESSURE IS COMING FROM THE MASTER CYLINDER DOWN BOTH LINES. It has nothing to do with what final pressure (either the same or different) is actually going to the front/rear brakes.
So if you think about it, installing the line lock solenoid either before or after the "proportioning valve" really makes very little difference, except that it will instantly light the idiot light on the dash when you set the line lock and then let off the brake if it's installed BEFORE the proportioning valve. The only other SLIGHT difference it makes is that when you go for the next brake application some amount of fluid must be used to move the piston in the pressure differential sensor back to it's "center" position to turn the idiot light back off. This probably accounts for the slight "spongy" feeling in the pedal upon next application. Thereafter everything should feel normal at the pedal.
Recommendation: put it after the proprtioning valve. But if you don't, don't jump off a bridge or anything becuase of that.
The annoyance of putting it after the valve is that the proprtioning valve usually has 2 separate lines directly off of it for each of the front brake calipers. That means you have to do more work to block one of them at the proportioning valve and then put a separate T in the line after the line lock. Just more plumbing to deal with. New2malibu's picture, above, is awesome for demonstrationg this- you can even see the T he had to put in after the line lick down on the frame rail.