Hello RjL...
Yes, your suggestion to use the syslog engine to run a program to set a custom property would be a good way to effectively raise an alert for a syslog message. I agree that this looks to be the more flexible option.
I am disappointed that this thread has been going for almost six years and there is still no built-in ability to generate an alert from a syslog message. The tricky thing I would imagine is that how to clear an alert based on a syslog message.
... simonevans