What's mindboggling about this boondoggle is that someone who is attending the event for which the garage is reserved might well end up driving all over campus trying to find a place to park, because the driver didn't think to say, "I'm here for the Alumni Dinner," and snugglegirl05 can't ask, "Are you here for the Alumni Dinner," or state, "The garage is reserved for those attending the Alumni Dinner."
If both sides, garage administration and university administration are happy with this, there's little you can do. Other than to ask if the University could include a parking pass in with the other information about the event. Or a secret password that the driver has to give the parking attendant.
As to the general rudeness of other drivers, who really just want to find a place to park (and I suspect parking for visitors is somewhat limited on campus), even if they are rude to you, the best thing is to keep fairly cheerful yourself. It would also help if you could hand out a map with the parking lots where they can park clearly marked on it. (I suggested this at a parking meeting at a University I attended--the administration couldn't understand that a *visitor* to campus wouldn't know the bizarre system of one-way roads or the fact that most of the parking lots were tucked away out of sight.)
One way to get around not being able to ask why they are on campus is: "If you would tell me what event you are on campus for, I can check to see if you can park here today." "Oh, just tell me what events are on today." "I'm sorry, I can't do that. But if you would let me know what you are here for, I can check." Although if they aren't saying why they are there at that point, they probably can't park in the garage.