I'm not much of a costume creator (now - but I used to be and *might* get back into it) but, my opinion / $0.02 is that things might depend on the context of the costume or the cosplayer, and perhaps even the historical context of the article which includes the swastika itself. Thing is, costumers are making scale models, or representations of figures we have seen.
Now, if I went to a maritime museum that had an exhibit of model warships through the ages, that's an example of a context where I think the fact that some model ships would have swastikas on them wouldn't upset many people at all. The political, genocidal, and racist message of the swastika really isn't the main focus of the exhibit. Rather, the model makers would paint the WW2 Kreigsmarine vessels with swastikas in places (fo'c's'le, naval ensigns, etc) in the same context as painting rising sun emblems on the Nihon Kaigun vessels. Message: "This is what these ships looked like in the era when they were in action." Same goes for captured rifle or maybe a soldier's mess kit on display that has a swastika or the Japanese Emperor's crysthanthemum on them. It's an authentic specimen - a piece of history with a story. The museum isn't promoting the politics denoted by the symbols on their specimens.
One cosplay question is, WHY does the character (in the story) wear the swastika? If you have a time travel story, where some beam or event nabs someone from a certain place and moment in time when that person was wearing a Waffen SS uniform, well, you're going to have this guy who shows up with a bunch of swastikas, much red black and silver colors, and a knife that says "Meine Ehre heißt Treue" on it. Now what? Depends on the story, to me. If the guy goes off vamp-killing with the rest of the Hellsing characters, then I'd think the racisim/genocide symbolism would be at least somewhat dissipated.
Also, as was previously posted, if the story character *does* act like an authentic racist berserker AND this is seen as a Bad Thing, and the story is 'how do we take this guy OUT,' then it's also not glorifying or condoning racism/genocide, it's presenting the 'good guy gang' with an opportunity to unite against evil, which is also cool.
Next, if the cosplayer appears as one of a SET of people who are like, 'all the bad guys of Hellsing,' or 'all the characters of Hellsing,' or a set like 'Uniforms of socialist tyrants of the 20th Century' (say, including a Khmer Rouge soldier, Jane Fonda a Viet Cong soldier, Cuban soldier, a Soviet soldier, a Red Chinese soldier, etc) - not really an anime topic, but again a SET - then it's kinda like the ships exhibit; a slice of history rather than a political statement. But that only works in the SET. If they move together as a group in the hallway, (such as on the way to/from a cosplay event) you get the SET. Once they disperse and the one swastika character is in an elevator apart from the SET - and the doors open on the 8th floor and some non-con people step in - you could REALLY hurt some people's feelings.
Last question: What are the odds that a real, living Holocaust survivor might be visiting the Hilton, outside the context of the con? He or she might be pushing eighty years old now, but would have been a teenager then, thrown into a box car headed for a place like Dachau, or Auschwitz, or Ravensburg ... Now how would you feel?
