Well, this fails on several levels for me.
First, the two day aspect of it. That's asking a lot of your guests, to spend two days celebrating your wedding. Most people are happy to give up a day, what with travel, ceremony, reception, and travel back home. But two days? That's the whole weekend for most people, and most people have chores and things to do on the weekend.
Second, the potluck. If most people are traveling and staying overnight, most of the food offerings are going to be things that can keep without refrigeration and that don't need to be heated up. Bread, for example, or cookies. Possibly a salad. Some enterprising guest may get some dry ice to keep a dish cold and a crock pot to heat it up, but I doubt most people will go to that trouble. Does the food need to be period authentic, as well?
Third, the costume. More time to find one, expense to buy or rent one, and the practicalities of lugging it to the wedding destination, along with the food for the pot luck.
I'd get out of the second day of activities by claiming prior plans. Those plans could be washing my hair and vacumming the dog, but they are still plans. And they are plans that would require me to leave right after breakfast to get home, no matter how enticing the potluck costume party might be.
If the Happy Couple had make their reception the costume event, with costumes optional, I wouldn't mind so much. But they are asking an awful lot of their guests for this wedding.
And if this is a new trend, to spend two days celebrating your wedding, I hope it dies a quick death. The Happy Couple gets a day for it to be all about them. Not my entire weekend.
This is, I think, a case of the Happy Couple thinking, "This would be so fun! Let's have a lunch with everyone in medieval/Renaissance/Star Trek costumes! Everyone will have a blast!" without thinking about the cost in time, money and effort on the part of their guests, who may not be so into costumes or potlucks.