Well users can create personal albums and stick all photos from an event in there.
Outside of the scenerio I layed about above using the additional cats function there is no way to have pictures all in different categories like you want and display an event with all Photos you want.
If you dont know where the photos are going to go how is the program to know? Do you follow me here. You would have to make some event categories and enable the additional cats feature so you can have photos where you want and have them displayed somewhere else too.
Your users can create an album for there event and use the additional cat chooser to also display them in your categories like you want yet have a album to show.