I went ahead and upgraded an AtlasO 40' AAR boxcar and figured the following would be the way to upgrade 90% of the rest of the RTR stock I have. I still have an AtlasO fishbelly hopper on my workbench to look at how I want to add the airhoses, but the idea shouldn't be too different.
The boxcar I did by soldering a small piece of bent .049" brass wire (slightly larger than the angle cock casting, but is a stiff enough base to use) to the end of the angle cock casting. The .049" wire was bent 90 degrees first before tinning and soldering the wire to the casting. Then I slipped on the air hose/glad hand to the piece.
The hole for the wire was drilled into the floor where it would let the end of the angle cock roughly parallel the edge of the coupler box. I used a #55 drill bit to drill the hole for the .049" wire. I did have to go a second time and slightly widen the hole, but it provides a very snug fit for the wire. This is so snug, I also needed to use pliers to push the wire into the hole. This should work on all plastic cars. Metal cars will probably need a more exacting technique, but I will find out with the fishbelly hopper's metal base.
Painting is trivial at this point. I do paint the angle cock handle silver along with the glad hand.
Here are some photos of the process:
