Authenticate with hash instead of plaintext password?

libpq lets me open a connection by specifying a password:
http://www.postgresql.org/docs/9.2/static/libpq-connect.html#LIBPQ-PARAMKEYWORDS

Is there any way to specify a hash of the password when connecting, instead of
providing the password itself? My Web UI asks the user for a PostgreSQL
username and password, and I want to avoid asking the user for the
password again later, as long as they have the browser cookie that I set.

I've looked at the source of phpPgAdmin, which should deal with the same issue,
but that seems to store the plaintext password in the session, which might even
mean that the plaintext password ends up on disk, though I don't know enough about
PHP to be sure.

I understand that libpq already sends only an MD5 hash to the
PostgreSQL server, when it's configured to use MD5 authentication.
But I don't want to have to provide a plaintext password to libpq.

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