How could someone use this? Well, I could decide to only target the rows with a low work factor. Since your entropy estimate is high for these rows, I can know that it's more likely they'll be 8 characters or longer and use a wider range of characters. I can likely ignore all candidate passwords that are shorter or that do not include non-alpha characters.
How useful is this? Let's assume 2 choices of work factor. Also let's assume strong passwords of length 8 have 96^8 ~= 53 bits of entropy and eak passwords of length 8 or less have 27^8 ~= 38 bits of entropy.
You just let me cut the search space for strong passwords of length 8 to to ~15 bits, and in a double whammy, I get to use bcrypt with a low work factor when brute forcing against these rows.
I'm not a cryptographer, and so it's entirely likely I've made some mistake here. But as a general rule, I think it is an _extremely_ bad idea to use cryptography in any way that exposes additional information about individual rows in a database.
Cryptography is not a place for innovative thinking. Even cryptographers need their cleverness to undergo exhaustive review.