Yes, output_buffer is not mapped as variable. You can search what fragment file has output_buffer setting in /etc/e-smith/templates/etc/php and then copy the file to (make the folders if no exist) /etc/e-smith/templates-custom/etc/php. Then make you changes in that file (the file you have copied to templates-custom). Save, expand templates and restart httpd-e-smith service:
expand-template /etc/php.ini
service httpd-e-smith restart
Remember: custom templates are priority over system templates.