A few data points from a recent experience:

1) Expertflyer, Travelocity, Expedia, etc don't show seats accurately - or at least the bulkhead and exit rows. They will appear as blocked off on EF, but they have actually already been assigned.
2) On some of the flights, the back half of the plane is blocked off until it goes to airport control. So you might have a good chance of getting a poor-man's lay-flat business row to yourself back there, depending on the flight. It's worth checking out at the gate, at least.
3) If you start with an AA-metal flight on your itinerary, online-checkin might never show up for the JL-metal/AA-codeshare flights further along. At least, it didn't for me. I got the JL PNR from AA, and was able to find my PNR through several reservation links on the JL website. However, when doing the OLCI, it told me it wasn't an option for that ticket/itinerary. My JL-metal fares were Q and S, for what it's worth.
4) Everyone is correct in saying that AA must assign the seats. JL could see them, but not assign them.
5) This was the most confusing and frustrating seat assignment process I had encountered to-date. But I did end up getting a whole row to myself on that flight. So the frustration was all for naught.