The positive stop on a 45 is already built into the shifter drum via the tension on the shifter drum plunger ball spring. Set tension for best results. Pa
this may be a difference between British and US English, but I'd refer to the mechanism on a 45 as the detent. A "positive stop" would be a ratchet-and-pawl mechanism by which the movement of the lever is translated into movement of the selector, and the lever is returned to its original position.
Likewise I'd describe the "police shifter" as the mechanism fitted to late Big Twins by which the positive stop ( ratchet top is US parlance ) was actuated by a lever bolted to the front down tube. It has a spring-loaded gate so that the lever will always return to centre and stay there without relying on the gearbox to do it.
I think the real answer to the original question, is that if there was any effective way of doing this at a reasonable cost it would be generally known. I'd go for the British gearbox option, not least because having 4 speeds makes a big differenmce to the general rideability of what is, after all, a quite low powered and fairly heavy bike.
If you mean a shifter like an Indian Scout, that's fairly easy. You will need a bracket welded to the downtube, a shifter arm and an adjustable link arm to the gearbox. As far as I can see that would give the worst of all worlds, in that you have a heavy remote shift lever, no positive stop, no positive location to tell you where the gears are and the lever is dependent on the detent ball to hold it in place, a job it was never intended to so. You also have a fairly hair-trigger control lever right next to your leg, which may be moving if you have a spring seat post.
Shoot, a man could have a good weekend in Dallas with all that stuff...