I did exactly what you did from JFK-NRT-JFK. Back in the day when you couldn't book a one way award, I redeemed enough miles for a first ticket, but on the outbound on JL I booked into biz and booked into first on the return on AA. The EXP agent called this a voluntary downgrade.

So, as the date approached, I called the EXP desk once a day to see if any first awards had opened on JL. Each time, I explained that I redeemed a first class award to NRT but voluntarily downgraded on the outbound. I then would say I was calling to see if an award in first opened. Throughout, I asked if there was a way to be "waitlisted" or have the process automated, but they said no, I'd have to call. Anyway, one morning I checked my res and I was in first on the outbound, too. I can only imagine that an EXP agent was nice enough to write my info down and check on it every now and then, since they all said automation was not possible.

Moral of the story -- be really nice and call AA every day to check if an award in first has opened up.