create your own MY_Log class, i.e. application/libraries/MY_Log.php
you can then extend the log class and write your own custom function:
class MY_Log extends CI_Log {
public function writeMyLog($entry)
{
// do whatever you want
}
}
then use your own function to write instead of the built in codeigniter one.
or you can overwrite the write_log function which is CI's default method, by defining your own method in this class.