Thanks Bernie, thats helpful.
After I posted the question, I was musing further; the flexation on the knees is exaggerated even further by the Vajrasana / Virasana / Saddle asanas, and whilst there is clearly some differing dynamics there because of the kneeling rather than standing posture, the knee stress must be higher than in a lunge, as you say making the "knee over ankle" instruction simplistic.
Thanks again.