Require entry of MD5 hash instead of plaintext password?

Is there a way to configure PostgreSQL 8.0 so that when prompted for a
password, the user enters the MD5 hash of his password, instead of the
normal plaintext password?

That is, when prompted for the password, instead of typing the
plaintext password as you normally would ("Foo", for example), you
would instead enter in the MD5 hash ("A8127A7CBLAHBLAHBLAHBLAH").