I have always received 40 TPs for domestic Class J flights - these have however always been booked as connections on an international itinerary and typically booked into D class, I believe. I do not know for stand-alone bookings, but they should book into the fare buckets BA list as "Business" in order to get 40 TPs.

Note that Class J is not really a business class product, but more of a premium economy (one less seat per row and you get a cup of tea, pretty much) - hence, maybe the reason why there are blurred lines between what BA lists as Business and what JAL is willing to sell as Class J.