Maybe it's just me, but why would you WANT to wear a costume to a gig? It's hot and uncomfortable, people will undoubtedly ridicule you...why not just go as yourself and enjoy yourself??? Although it would be quite entertaining to see the moshers pummeling the crap out of Batman Akerfeldt, or whatever the hell you have in mind...
It would be cool if everybody in the crowd were wearing Michael from Halloween masks moshing and headbanging. Mikael would look out and be like:zombie:?
Obviously your allowed to, but your allowed to shit on your floor as well. Doesn't mean it's a good idea. Plus Opeth themed characters are a pretty small window I mean; jack frober, ghosts and melinda, not that much to work with. Plus as people have said, there is the impracticality of being hot as fuck and most likely getting bashed.
It might work, but then again... it would be hot as hell in a costume surrounded by dozens of people and if not being cabable of moving one could easily get claustrophobic.