You can still check seat maps (and inventory for standard buckets in Japanese) at

http://www.airtravel.tk/

The seatmaps will give you a general idea of how full the flight is in the class you are interested in but it will not be a very accurate indicator of how many seats are left. But the inventory and thye seatmap can be used together to get some reasonable idea of the situation. If you see C0 in the inventory and all seats in C assigned in the seatmap or the seatmap comes up saying the map is not available at this time for C, you are most likely not going to get an upgrade.