Nova filter based on VM metadata?

Hey, all. I'm looking for a way to to filter to a particular aggregate based on arbitrary VM metadata. (E.g., maybe FACTER facts from Puppet that we pass to the VM as metadata.) Could someone point me toward documentation that might outline how to accomplish that? As I see it, there's really one thing I need to understand -- the rest I can probably glean from extant filters: how (or if) a filter could access a given VM's metadata from the API creation request (if that's even possible).

Thanks!

-Ken