Run a SQL statement in MySQL on each connection
Found it.

The parameter is named init_connect and will run the defined sql for each connection, except for those with the super privilege. No result sets are produced by this though so selects are not recommended for this.

The big cavet I see here though is that access to the objects is based on the connecting user's permissions, which make sense.. though it does make an 'autorun' style stored procedure harder to implement as that procedure needs to be accessible to everyone, which could cause problems if it's written with the expectation to be executed only once per connection.