If you insist on validate_db_connection, then there is only one way to solve this :)
The paths to the certificates are hardcoded in the libpq C library, which is used by psql.
So, you have to store the certificates multiple times or symlinking the contents of the .postgres folder to a different location.
This should probably handled by the puppetlabs-postgresql module.
Let me know, if you come up with any other solution.