Seems like a no-brainer. I've been looking through the commits, but couldn't find it. I think classic has a defense along those lines.
The rule for classic is to directly limit the amount of hashing required. If your block does more than 1.3GB of hashing, it is invalid. I assume that the 1.3GB limit is sufficient for a 1MB transaction. Ideally, any valid version 1 transaction (so less than 1MB inherently) should be valid when rules are changed.