Yeah, there's very little surviving documentation about heater shield use. I have a feeling they were used more on horseback than on foot, but that's a personal guess. And in terms of using a shield and a lance on a horse, there probably wasn't much worth writing down about the shield.
There's some documentation on sword & rotella (a round shield, like
this one from MRL), I think. Presumably much of it can translate over. And it is a bit different than how SCA fighters typically handle it. In the classes I took on it, the shield was used mainly to close off a line of attack, and was held somewhat outstretched.
I think Rodney made a good point about how much the SCA combat has evolved to the SCA's rules. This is only natural, as that will happen in any game. The "wrap shots" are similar to some maneuvers in HEMA, but there's much less reliance on that in a real fight. Partly because doing so requires you to be close and to expose your arm. SCAdians also have the luxury of holding the shield high and leaning back, exposing their lower legs since they're "off target". Also, sometimes the shields are absolutely huge, being made of modern lightweight materials (aluminum). IMHO this becomes a game of who can reach over the other's shield the best.
Granted, not being an SCA fighter, I'm not an expert on what they do.
But to be fair, there's only so many ways the human body can move, and even through trial-and-error, similarities will develop, just out of what feels efficient to do.
