write to syslog using iControl?

That is correct, at this time there is no way to issue commands on the BIG-IP. We've abstracted system commands through the APIs for security reasons and have not yet had a request to support syslog messages directly through the API.

If you need to update the syslog remotely, then you could, as Mohamed suggested, remotely issue a command over SSH.

Another option could be to create a dummy iRule with the log message in the RULE_INIT event. By adding the iRule, the RULE_INIT will get triggered and then you can delete the iRule when you are finished. The LocalLB::Rule interface contains the methods and a simple rule like this

when RULE_INIT {
  log local0. "LOG SOMETHING HERE"
}

would work.

Now, you probably wouldn't want to do this every second as that would cause some overhead on the system, but if it's a infrequent event, then this approach shouldn't cause any issues.

-Joe