I know of a freeware command-line tool that returns an errorlevel value of 0 or 1, based on whether or not the amount of free space you specify exists. This could be put in a command script and run either manually, or by Task Scheduler. Then, based on the errorlevel out of that, you'd simply run the Disk Cleanup utility (cleanmgr.exe), and empty the Recycle Bin.
It does do what you request, even if it seems kludgy...
The utility I mention is by Douglas Good, and is called IsFree.exe. Doug seems to have fallen off the internet forever, but I have a copy of this utility, but I won't bother posting it since I doubt it is in line with what you want.