Logging only mySQL logins on heavily used system
I really wish I could take credit for this, but I can't.  I chanced across this while thinking about your question: 

http://www.fromdual.com/mysql-logon-and-logoff-trigger-for-auditing

It never occurred to me to call a stored procedure from init_connect... but I (somewhat enthusiastically, I might add) tested this solution before posting it here, and found that it works exactly as described, on off-the-shelf MySQL 5.5.30.  

It logs every login of every user who lacks the SUPER privilege.  This is a bit of a limitation, except, of course, for the fact that a SUPER user could manipulate the audit table anyway.  

To capture the logouts with similar logic, you have to modify the server source, since init_connect does not have a built-in counterpart that gets executed at the end of the session.