i haven't tried it myself, so i am just brainstorming here - and to be amongst the first to post in 2012 :)
1.
inside "list_articles" you can set ignore_publication="true". if there was a chance to check for the publication ID in the constraints for "list_articles" this might be another way to do it. but i could not find that in the documentation - possibly holman from the Newscoop team can give us an answer?
2.
again, inside "list_articles" you can set ignore_publication="true". then, inside the "list_articles" routine you can check with IF. this should work, but would confuse issues like list index - and therefore also create problems if you want to set a specific list length or use pagination.
3.
following idea 2, you could go enterprise and set a smarty variable with increases by 1 inside the IF routine and could then be used like list index. however, pagination would still not work and at_beginning and at_end inside the list would also not work properly.