I don't really want anything exclusive other than than the physical stuff, I want (and I want everybody else to have) a .exe that just installs the game and I can play it the day it comes out, and 20 years later and find no difference regardless if I remember my old account logins, installing it on a different machine, whether activation servers are still up, etc.
So long as I can do that I'm all cool, and this doesn't sound like it effects me, I'm not going to vote against because I think it's good that you are looking for a non-gameplay altering thing to give out and hopefully it will increase some $15 pledgers to $30, but at the same time I can't vote for it as it's not something I want, for something to be exclusive to us. I'm going to abstain.
Just a suggestion as well, if you intend to keep things DRM free then these things will inevitably find their way over to the people who didn't pledge. What about embracing that? Maybe put them in the game for everyone, but hide them away somewhere and give out a sneaky code to the backers so the regular guys have to hunt them down and find them for themselves, either in-game or out of the game. Preferable in-game. That's also a nice hark back to the days of cheat codes or secrets, and the backers will be the popular kid with the cheat book