Hello,
Recently became a STE+ pax (UX Gold) and from my new experience, I am able to select a seat before check-in starting row 2-6 (Same seating map as Flex fares on Medium-haul tickets).
Flight In 3 months: row 2
Flight in 1 month: row 6
So it depends of how big is J class, the availabilities maybe and proportion of status pax on some routes maybe?
There is no particular blockage (just middle of the plane and some seat at the back on the left side).
AF MH