I may be being a bit stupid here, but how do you advance the slides on that link? I've never been able to figure out how to use InfoQ presentations. I must be doing something wrong...
How about a "silent shadow" system? When an account reaches a slope on the negativekarma-time graph, just make it so that a very small, random group of users are the only ones who actually see what they posted. To everyone else, it would be as if they hadn't posted at all. All the while, if what they do post gets a large percentage of upvotes compared to the amount that see the post, shorten the "shadow silent" accordingly.
It will not be made known to the user that their posts cannot be seen. They will have no reason to use alts, and we will have no reason to IP tracking, which could affect non-troll users.
Convenience is what you should be going for. If you're able to generate a short url for convenience, the chances of someone signing up later rather than saying "Signup just to try? fuck it" are higher.
Hell, when generating the url, just make a call to the api at http://tweak.tk so that you can display the short one side-by-side with the long one. Provided your codebase is well formed, a change like that should not be astronomical.
(P.S. Tweak.tk is really awesome. At least check it out.)