I was thinking of a system where the mod generates a hidden file inside it's jar on the first run, containing some kinda identifier. Then the mod would crash if wrong computer was used. The problem is that those websites probably doesn't even run the files before mirroring them. About the file hash, I like the idea.Perhaps I can make the file edit done serverside (if Google sites allows me to encode such a thing into a link)?
I was hoping to go for the former, but I doubt that sort of data gets encoded anywhere. Another idea is a file hash, but that does not solve the problem if they ship the original file unmodified (even though they do often add spyware and other malware).