I was on a 763 flight from DFW-LAX on AA about a month ago (equipment change from 757!) and was seated in the F cabin. There are two lavs in front of the cabin, the one aft of the door was in use, so I chose to use the lav directly by the cockpit. I almost thought the FA was going to tackle me, she immedietely jumped in front of me, blocked my access any further, and instructed me to use the lav behind me. I told her that it was occupied, and only then would she let me access the forward lav. Certainly, a tense moment!
I haven't experienced that prohibition since then, so it must have been an FA focused on safety first.