I know this is a little out of the realm of LEMs purpose, but can you use the Windows connector to monitor disk usage via a filter or rule? Just wondering...
One way: Windows does have some default triggers that fire when the disk is "approaching capacity". You can adjust the threshold (I think by default it's 10% free?) in the registry and it'll fire sooner.
Another way: If you set up a Performance Trigger to log to the Application Log, LEM will pick that up, and you could create a rule/filter based on that.