upgrading only urgency=high packages

[Dan Christensen <jdc@uwo.ca>]

Is there a way to upgrade all currently installed packages which have
had an urgency=high version uploaded to the archive since I last
upgraded?  (And any necessary dependencies, of course.)  I'm thinking
of this for the unstable distribution.  The idea is to frequently do
such upgrades to get any security fixes, and less frequently do an
entire dist-upgrade.

(I know about testing, but for the machine in question I like to
stay current with unstable most of the time.)

I'm guessing that the information about the urgency fields might not
be available except in the changelog, so it might be necessary to
download the package and have a script look through the changelog.

Could apt-listchanges be hacked to do this?  Or could apt's new
pinning mechanism help?

Is urgency information stored anywhere besides the changelog?
How do the testing scripts do this?

Thanks for any suggestions,

Dan