So I've just returned from a trip to Japan where I made extensive use of JAL's "Japan Explorer Pass". Each segment is a domestic flight, priced from 5,400 JPY (£36.94) to 10,800 JPY (£73.88) depending on route. The great thing about the pass is that each sector can be upgraded to J at the airport (subject to availability) for an additional 1,000 JPY (£6.84). (F class upgrades are also available on some routes, but this is not good value from a TP run perspective -- J class is the sweet spot).
I was able to obtain a J class upgrade on every sector where the plane had a J class cabin (four out of five), earning 40 TPs each. I earned 20 TPs for the sector where there was no J class on board and had to stick with Y.
If you were to be strategic about this, I reckon there could be decent earning potential. E.g. suppose you strategically choose lightly-loaded flights between Tohoku and elsewhere on planes with a J class cabin. By default, you'll earn 20 TPs for £36.94 (£1.84/TP), with a decent likelihood of earning 40 TPs for £43.78 (£1.09/TP).
Obviously you would need to factor in a trip to Japan in the first place though -- but if you're planning a trip to Japan anyway it could be a nice little earner. It's also, in any case, a very cost effective way to travel around Japan.