Is it possible to make a certain party required for an event?

Heh, I was actually gonna say event command 99 to open the party select screen, then force the characters, when you enter the place...

On second thought that would only work IF you had recruited the characters already, if not, they'd be added anyway, I think.

Yep, DE would probably be a better way to do it. Check if they are in the party and reject if not.