I'm really surprised, or maybe I'm not, at the number of suggestions that are being offered to avoid complying with the wishes of the bride and groom. All of them, beyond merely asking for permission and accepting the answer given, involve manipulating the truth so it doesn't look like what it is, a lie.
Would you guys be as forgiving if the situation was reversed and someone did this to you because they thought it was in some way better than what you wanted? What if your Best Man substituted those ugly square toed shoes for the "proper" shoes that you asked him to wear and didn't tell you he was going to do it? Or made up some story to cover for his deception? It's the same thing that some are suggesting.
If you don't want to be a part of it, then don't. But if you do and tell your friend that you will, be a friend he can trust and go along with his requests even if it isn't what you would do if it was your wedding. As important as some think clothing choices are, there are some things in life that are more important; like friendship, relationships, honesty, trust.
Just a thought.