It sounds like you know what you need to do, and Community Auth makes this easy, as long as you use the db_tables config file. You could just change the value of user_table to something else, like ca_users. Then just rename the table in the install script to ca_users. When you want to use that table, you'd still use $this->db_table('user_table'). There's nothing "custom" about doing that. Community Auth was created with this flexibility for cases like this.
