A well aimed shot with a ball peen hammer would do. Not distorting the flat mating surface of the header would be a concern.
1 more thought as if you don't have enough of them. I've been looking at the pic and it appears there might be enough room to get a screwdriver blade between the nut and head pipe. If that's the case maybe you can slightly pry up the pipe (indent) it enough to get the room you need to get a socket on.
So, how soft and pliable is the head pipe? These ideas are all great but I feel like it's going to be a lot harder to get a good groove or indentation in that header than just applying pressure with a screwdriver. I also don't want to just start banging with the hammer and mess up the mating surface as Bodeen points out.