I am pretty sure this would require a fairly significant re-write.

I believe the flag is currently built off the lastviewednewestnodes var and the logic is display stuff if newer than that.

what you would propose would require the ability to mark a specific node as viewed, not a list of nodes that post-date lastviewednewestnodes. This would probably be implementable via another table but I would bet that the database overhead of checking this for every node and every user would make it unworkable