In general, what you're asking about is called a "salted hash". I don't understand enough about Disqus's system to say it would definitely have prevented this vulnerability.
A salted hash only slows down brute-force attacks and dictionary attacks. The salt is still stored with the hash, so you could still eventually match the email address with the hash. Instead of hashing each email address once and comparing it with all of your collected hashes, you'd have to hash each email address using every salt until you found a match.
Sure since they're using MD5 then a salt wouldn't solve their problem, especially if the salt were also part of the url. And let's not pretend that the url itself is somehow secret: there are many ways to collect those, particularly if specific users are targeted. Usually when people are this boneheaded about hashes they're trying to save storage space, but I can't imagine that storing a separate random identifier would add significantly to Disqus's storage.
