I'd say its possible, but unlikely; and you'd hope its not a major factor in practice.
When you assume that the attackers would have, or could get, at least one account which they know the password for, then if the hashing scheme is something they'll be able to crack en masse, then they are probably going to be able to figure out what the hashing scheme is, from their known plaintext.
When you assume that the attackers would have, or could get, at least one account which they know the password for, then if the hashing scheme is something they'll be able to crack en masse, then they are probably going to be able to figure out what the hashing scheme is, from their known plaintext.