No problem. Park's still open to non-MVMCP at that time. The only problem you might have is leaving... Main Street gets very crowded when day guests are trying to leave and party guests are pouring in. I'd take a "sneak route" to get out of the park after you eat!
If you're in the park on the regular ticket, you can dine as late as 6:45. But you'll be directed to the exit when you finish. (although I am hoping for a peek of the party that I'm too cheap to attend!)