querying rpm database for installed packages signed with specific gpg key or from specific vendor?

[Gabriel M. Elder]

Anybody know of a way, using package management tools (or otherwise), to
query an rpm database for a list of all installed packages from a
specified vendor and/or matching a given gpg key? For instance, i want a
list of all freshrpms packages that i currently have installed. I figure
the simplest way to find out is to (politely) ask the rpm database for a
list of all installed packages from this vendor, and/or signed with that
gpg key.

I just looked at the man pages for yum and rpm; either i'm missing
something, or they currently don't have a facility for doing this. Does
apt or smart provide this capability? Should this be a bugzilla RFE
feature request for yum or rpm?

- gabriel