MGM is unique in the fact that they like to throw in a Fantasmic a half an hour after the park closes. Haven't figured this out but they do it a lot (EMH or not). If it is listed on the regular park hours, which it is the day you are asking about, you will be able to go to it regardless of EMH. You just won't be able to do anything else once the EMH has started. Guests that are there for the EMH will have wristbands to signify that and will be asked to show them for the rides and shows that are open for them. After Fantasmic is over they will basically make sure that all guests that were there just for the show, that do not have wristbands are directed to the park exit.