I don't see how this would really work. As soon as one core pulls in a cache line, all the other cores know about it, so somehow you would have to roll back all the coherency messaging as well.
You can in a many-world interpretation of quantum mechanics. On a branch speculation, split the universe. Follow one branch in one universe, the other in another universe. If the CPU realizes that the speculation is wrong, destroy the universe (this is left as an exercise for the reader). As a bonus the CPU will have 100% speculation success. Unfortunately I think quantum mechanics itself might allow for side channels between universes.
