Hi bklink. I had an avios booking via ba.com for a domestic JL flight earlier this year. The JL MMB site is here btw, but you can't seem to select a seat until 2 months out. Click on "My Reservations" button to look at your booking.

I think you need your JL PNR as well - different to the one BA give you on the e-ticket. If you do you can get it from checkmytrip.