I don't really like the idea of publicly displaying videos (or screenshots) of wallhacks, even if they're just demo reviews. The reason being, it might give people the wrong ideas. People might think you're actively using cheats. People might think it's OK, and that they should use cheats, too. Some people might not even be aware these types of cheats exist and start looking for them. All in all, it just seems like a bad idea.
As for a built-in demo-viewing wallhack, I have considered this, but it seems really risky. If I don't account for something correctly or something, I've basically included a "free" wallhack built into the game. It would also make it trivial for people modifying the engine to remove the demo-only check, and, while it would be detectable, I don't want to make it TOO easy. At least if some work goes into it, people have time to think about the ramifications of their actions.