JL holds back a very large portion of the Y cabin until check in time. So it is quite possible that if you check in as soon as check in becomes available, you will be able to snag a decent seat.

Also, I don't know if this is still true, but in the past JL also held back a large number of seats from web check in, so there were a lot of seats that could only be assigned at the airport. So if you still don't see anything good at online check in, try to get to the airport early and change your seat at the check in counter.