This is a situation I've encountered quite often, albeit almost always on the 2-class DXB-KHI-DXB route where 3-class aircraft seem quite commonly substituted. I've always been able to pick seats in the F cabin online (both MMB and OLCI): they've merely showed up as 2-2-2 configured seats in the J cabin, just situated ahead of the standard 2-3-2 seats (no bulkhead/divider shown). Now that I think about it, the gold plastic may have something to do with this - not quite sure.