Currently with logmon or other log monitoring tools as each basically check a block of test or a single line for a string
Where it is possible in THEORY to write a Regex to check for something that is NOT there it is very difficult and usually very IO intensive to do all the checks necessary.
That coupled with lack of a time make this very hard if not impossible.
I have talked to other clients who use a script to check for a string and keep track of the time.
If an alarm needs to be generated based on the checks int he script either use nimalarm to generate the alarm or update a log file that logmon is watching to generate the alarm.