Off the top of my head, one way to do this without rearchitecting everything would be to insert a new record into a very simple in-memory temp table. Every minute, update the appropriate record in the real table by count(*) of the temp table and clear the temp table. As it's being used for a non-critical counter, data loss in the event of a MySQL restart isn't a concern.

That should be sufficient for something of this scale and only requires a minor update to the client SQL, ensuring the temp table is created when MySQL starts, and executing one stored procedure on a schedule.