Is it possible for Syslog-ng PE report on alerts?
For example if a file size zero" bytes in size or the file last modification time is less than X minutes.
Syslog-ng PE does not have a way to report on alerts, this is usually the job of SMNP software.
SNMP software can be used to trigger alerts to the SMNPTrapd service that can then be read by Syslog-ng using the SMNP source.
Alternatively a Cron job could be used to schedule a script to run that queries the file and then either logs that information to a file that Syslog-ng can read from or email alerts could be configured to send through the Linux OS itself.