Linux memory thresholds

I'm sure someone else has had and solved this problem, but short of writing a plugin to handle it, I'm at a bit of a loss.

Linux "free memory" is actually the sum of three components in SNMP and Zenoss - real memory, cached memory, and buffers.

When setting up a memory threshold in Zenoss for a Linux server, you want the threshold to be against the sum of those three, not just real memory (because a very active server could have virtually no "real memory" free because it's all tied up in gigs of cache and buffers, which is a completely normal situation and that memory will be reclaimed by applications as needed).

Unfortunately, Zenoss only allows thresholding against a single datapoint, making it essentially useless for thresholding memory usage on Linux servers (Sun, for instance, doesn't have this problem as it reports free memory in one "chunk").

Has anyone else already solved this problem? It seems a shame to even consider writing a plugin to handle this, having to deploy it to each server, and wasting the additional CPU cycles to return data that Zenoss already has.

Rodney