The "armoured lord on the ground" experienment isn't a brag. It's an experiment.
I expect that if the "peasant" is sitting on my chest and attempts to go for the groin, I'd just buck them off like I would during combatives when someone gets off balance. When they lean back or twist to get a shot to the groin, they will be off balance. When that happens, I should have my heels as close to butt as possible. Then as the weight shifts I "buck" the "peasant" off to the left or right and achieve side control as they tumble or achieve my feet.
If the "peasant" is in the classic combatives guard where my legs are locked around their torso, the groin shot becomes really difficult. They stand an amazing chance of gutting themselves. I'd block the eyes with one hand and the groin with the other, attempting to control the dagger and get them to gut themselves.
If the "peasant" is in side control I've got a lot more concerns IMO, and side control is far more realistic as a starting position. Now their head is blocking my view of their other arm. The solution is scoop towards my own groin and lock them in place.
And, he would have to stab through the mail drape and steel codpiece to get the groin. It's a difficult target IMO.
I think the A&S and marshal would approve of this test, if done appropriately. I'd require eye protection and a gorget for my opponent, just in case a piece of armour goes where it shouldn't.
And then we'd have to look at varying weights of peasant. I doubt I could move if Blackbow had any sort of control. It would be like moving a small truck -- I'd have to convince the truck to move willingly.
With respect,
-Aaron