I've given up trying to get my FF # in my record for reservations made through corporate travel. If I remember, I give it when I return the car. The other times I just give up on it. Just another annoyance renting from Avis.

Edited to add:
I just noticed that Avis.com allows you to add the FF# to past rental agreements, now I'm waiting to see if they post. Finally a nice customer service item from Avis!