[gojomo]

Doubt that'd be worth the effort/storage, even in the case of a weak, unsalted hash like MD5.

Take N to be 10, assume 7 bits per character. Then all 10-character passwords fill no more than 2^70 of the 2^128 MD5 space. Any 11-or-larger character password then has a less than 1-in-2^58 chance of colliding with any shorter password. (That's how much larger the full space is, from which each longer-password hash will be drawn.) That's 1-in-288-quadrillion for us decimal apes.

The service would probably never deliver a useful warning before MD5 falls completely to a preimage attack.

The analysis for such a service only gets harsher for 160/256/512 bit hashes.