Interesting premise, but it does fail to differentiate between "something's gone wrong, stop this person for five minutes" blocks, and 'punishment'/'cooling-off' blocks. Hmm.
Making it an option on the block page (in place of the "anon users only" check box would work well) would fix that. When blocking vandals, you want to unconfirm them, when blocking someone for a minor bit of incivility, or a 3RR violation, you might well not want to.
It used to be "the most recent n% of users", and was used to determine who could move pages. Then we enabled semi-protection, and it seemed sensible to reuse this definition as the target group. At about the same time the definition was changed from something based on number of accounts to a fixed three-or-four day limit, which was roughly the same period of time but less of a logistical hassle (and harder to game by flooding new accounts)
Excellent!