I've been trying for a while to figure out how to get my tbird filters to mark posts that are cross posted. I finally got it to work! I'm not sure why it wouldn't just work when I set up the filter, but I had to right click on the newsgroup name, select the newsgroup(folder) properties, click the Repair Folder button, and re-download the group posts, and ONLY then did the filter I set up work.
So, now any post that is cross posted is tagged "Crosspost" in light grey and is marked "read".
Anyway, thought I'd mention that it seems Tbird has a bug that won't necessarily respond to this particular filter unless the groups folder is repaired/reset.