If a friend got the event Pokemon, perhaps you could arrange a trade with him/her.
I'm not 100% on this one, but maybe that Pokemon will be distributed at another event, you never know. That's what happened with Victini, but they were two different Victinis.
You can keep yourself updated here:
Also, there are Dream World events. If you miss a Wi-Fi event, maybe, just maybe, there will be a Dream World event for that Pokemon.
If anyone has corrections, please tell me.