The only other JL 7th freedom flight is JFK-GRU (so not too useful).

Does anyone have experience of MEX a a same-plane-turnround point? Is international transit possible or are there any visa requirements?

And the NRT-GRU I do see as potentially useful as a "segment saver" - being able to do Asia to S America with only one of the twenty segments.

International to International is ok, you stay airside and no visa required.

True, but only useful if you want to go to GRU (not the best fun destination in SA). Otherwise you burn a segment to get to SCL or EZE, so it is 2 segments NRT-GRU-EZE -- when you can just do NRT-JFK-EZE or NRT-JFK-SCL. No segment saved, and going to Brazil can be expensive for some people. But the JL flight to GRU does offer A year-round, so it is more of an A-saver (and connects to the BA A-service out of GRU).

GRU is not on my radar either, so I don't see myself taking that flight any time soon, but NRT-GRU-SCL is a better MR, compared to any other NRT-US-SCL.