Custom syslog-ng facility

I've got a log statement in an iRule which generates quite a bit of information to the local0 syslog-ng facility. Instead of logging to the local0 facility, I'd like to isolate these entries from local0, and instead put them into their own log file.

I was thinking I could do this by using a custom syslog-ng facility like this:

iRule:

   
   when HTTP_REQUEST {   
     log local100. "some log entry"   
   }   
   

Then in syslog-ng.conf, I'd have something like this:

   
   filter custom_filter {   
       facility(local100);   
   };   
      
   destination custom_log {   
       file("/var/log/customlog");   
   };   
      
   log {   
      source(local);   
      filter(custom_filter);   
      destination(custom_log);   
   };   
   

But the LTM log indicates local100 is an invalid facility. Is it possible to define your own syslog-ng facilities like this in some other manner?