I'm actively considering Splunk
This is the direction that Professional Nagios is going for interrogating logfiles.
nagios.org/products/enterpri ... ons/splunk
There's a free edition, which you'd have to integrate yourself..
Just as a WAG without thinking about it much, one thing you could do after getting Splunk going is configure it to do something like
put a certain output file in a given directory, and then use check_file to see if there's a warning file in the directory.
I think Splunk itself has all kinds of warnings, alerts, abilities to do X upon noticing thing Y and so on.
but if you want a tie-back to Nagios, that's one way to do it, albeit probably not the best.
There's probably something way way simpler that I just haven't read about yet.