An Example of Proprioception

…..myself and another one of my athletes experience very dull but evident discomfort on the lateral aspect of the knee joint during a seated external rotation stretch…….this is when the stretch leg is brought into external rotation, abduction, and flexion on top and across the other leg. There was nothing acute that occurred to either one of us, and I presume it could just be an inflammed IT-band or LCL since we are both bow-legged? Lateral hamstring or gastroc tightness?

Yes, this position can traction or torque the LCL, lateral capsule, or Gerdes Tubercle where the ITB plugs into the tibia.
It’s not a position I recommend on your back, but rather standing progressing to the Pigeon position on the floor.

I wonder if that degree of L5-S1 extension is worth it.

I don’t think hamstring tightness would be a huge impact here since the knee is bent, but if the tendon is thickened, it could add to some soreness. If bow-leggedness was really a factor, a lot more athletes would have reliable challenges, and I’m not sure that’s something that is seen often.

Something to consider with this FABRE position stretch like what you are describing or the butterfly position, is that if the lateral knee or thigh is not touching something from the floor up, I think this is a problem, at least an inefficiency.
When you assume this position, there is tightness medially through the groin, or potentially laterally in the knee because the position is proprioceptively threatening. The body thinks that the leg(s) is falling and tones the ADDuctors or other muscles to prevent the perception of free fall.

This is a bad Pigeon.

So in the standing and lower Pigeons as well as butterfly, never try to crank the leg down if it doesn’t go on its own. Rather, build the floor or table up to the lateral knee that you are complaining about. Give it a floor and the proprioception to let the hip muscles release from protective mode. You may find that you can get even lower into the rotation with less tension around the hip and knee. This is because the brain IDs that there is safety underneath the knee. This is another of the many examples of proprioception.