Progress - I found SQL that returns the data I'm after (MAC, username, timestamp and certificate serial (without colons) - see below). However, placing this query in the export filter doesn't appear to work - I see no data in the logs now. On top of that, when I select a Data Filter and save the export filter, it doesn't save the changes. Not sure what I'm missing now.
SELECT t1.user_name as userName, t1.host_mac as macAddress, REPLACE(attr_value,':','') AS certSerial, t1.timestamp as timeStamp FROM tips_session_log_details t2 JOIN tips_dashboard_summary t1 ON t2.session_id = t1.id WHERE t2.attr_name = 'Certificate:Serial-Number' AND t1.host_mac = '%{Connection:Client-Mac-Address}' ORDER BY t1.timestamp DESC LIMIT 1