I'm not sure if it's required but it's certainly good practice to tell the airline about your infant when you make the booking. I always call up right after I book the (adult) ticket on the web.

Also, while I'm not sure about the actual policy, I've seen rear facing seats installed in US airplanes and I've even done it myself with no issues from the FAs. Getting her to stay in the seat for more than 30s is a different matter, though. Why not give a call to the airline and ask what their particular policy is (then post the answer here for our edification).

Michael