I'm not aware of any options that can be added to the symbiosis-httpd-logger process to fix your problem. There are two implementations of that process, both FOSS, so you can get ahold of them and modify the source code so that they flush the file handles regularly. I'm guessing that's where the buffering is happening anyway.
You also can bypass using the symbiosis-httpd-logger process entirely by modifying the site configuration in /etc/apache2/sites-enabled (or sites-available). The intended purpose of symbiosis-httpd-logger is to mirror your sites logs into their respective log directories and set the file permissions correctly. You could edit the CustomLog line in the Apache2 configuration(s) and just write the logs directly to where you need as long as you don't mind the file permissions changing.