Sometimes (rarely) a CM will let you in if you don't have a FP (for example a couple of weeks ago we had a FP+ for Rapunzel (who 99.9% of the time meets with Snow White.) My daughter was dressed as Snow White and we planned to get pics of the two of them together. Well, it wasn't until we got into the room (and after having scanned our FP+) that we realized at that time Rapunzel was meeting alone (no Snow White or any other visiting princess friend.) Well, I was pretty disappointed that I 'wasted' my FP for that time and couldn't get another one for that particular attraction that day. An hour or so later I went to the FP+ entrance and told the CM what happened and she saw my daughter dressed as Snow. She told me Snow White was meeting then and let me through the FP+ entrance without a FP+.
So that said, it *COULD* happen that you have a particularly nice CM who will have mercy on you and feel for your situation, but given the frenzy and insanity that the Anna and Elsa line has created thus far, I personally wouldn't count on any CM being able to do anything to help you. Too many people will be fighting too hard and waitng too long to see those characters for them to bend the rules a little. Just my opinion.