I can't really speak for the bluegrass pedagogy very much, because bluegrass is a very American South style of music. I live in Australia, so...

But I think from what I've heard there's a lot of first hand teaching like there is in the gypsy jazz subculture. The technique is more varied, as you've pointed out, but if you're doing the orthodox bluegrass rolls (which can be either pure alternating or repeating down down up) your technique has to sort of converge on a curved pick trajectory because there literally isn't any other way to play the rolls at speed.