I run/provide programming to the most AMV rooms of anyone and on an average, about 350 AMVs are compiled for any one con. Also, we don't always play exactly what we plan and sometime play other stuff too on the fly and also have a large pools of extras. Pretty much if it's been released publicly, then it has the potential of being played as I like to showcase as much work of people in this community as much as possible!
If I was to try to hunt every single person., group down to ask to for permission, then I would not have time left to do all the work for the rooms themselves as then would amount to about 1000+ videos and probably 500+ people/groups to try to contact. However, if we do show a special Project or analyze something in a panel then just outright play it, etc, then for that I look at contacting the creator.
In the end, I look at it this way, if it's publicly released its fair game... If anyone does not want any of their videos played for what ever reason, then they need to let me know and I'll take them off the list.
It is my honest opinion that playing AMVs at Cons to a live audiance is they way it was ment to be in the first palce

ANd the audience sure loves them! Hell, at most of the cons I run the AMV room has become the biggest room and fastest growing event!
Vlad