Note also that osm.org doesn't use osm2pgsql for expiring tiles, they have a custom script that doesn't look at relations. The reason for it is that some edits to large relations can expire a lot of tiles IIRC.
I can't recall where the script is but it's some ruby script in OSM SVN.
Yes, i already adopted it for out multi-style-environment on cassini and placed it at [1], but on cassini it was very slow with this much styles.
Peter
[1] https://svn.toolserver.org/svnroot/mazder/diff-import/tile_expiry/