Sieve is an RFC email language. There's no reason Sieve recipes couldn't be applied to a corpus of existing email; that's an implementation detail of FastMail. Sure, it isn't necessarily the same as pre-receive (for example, anything you discarded the first time isn't going to show up again, and any manipulations made the first time aren't going to undo themselves). But it could be done; it's not really Sieve's problem.