What is the reason why nobody really ever tried to optimize jpen blades for RPB? I think dang qiu actually tried jpen rpb with a special blade for this for some time but then went to cpen later. What is the disadvantage of jpen for RPB (apart from blade weight and cork on back - but that could be both changed)?
Nothing there is no disadvantage to using jpen + rpb, perhaps maybe tradition. I think this is a big misconception. They are still both penhold paddles which means you can still rpb with it, just like how you can RPB with any other paddle.
I actually know more JPen players locally that play with rpb.