Dovecot X.509 authentication problems (cn mapping)

Hi all,

I'm trying to set up Dovecot to allow authentication with only X.509
certificates and hit a snag. The CN is set to the user's full name,
f.e. "John Doe", instead of the username on the system "jdoe".

I looked at the userdb/passwd configurations and didn't find a method to
perform a rewrite or a user mapping that would allow me to override it
short of using an SQL userdb and overriding the user field.

Is there a way I can "map" the usernames to system names? e.g. given:
c=US,st=California,o=Yoyodyne,cn=John Doe
I want a mapping such as:
/etc/dovecot/cn-mapping.txt:
jdoe: John Doe
jsmith: Jane Smith

etc.

Re-issuing certificates will be a problem (think SSO: single-sign-on), so
I'd rather tweak Dovecot. If there's no way to "map" usernames aside using
an SQL database, I'll set up an SQLite instance for it.

Thanks.

P.S. email address rot13'd