wow, I have not seen one of them in ages. I do not think it was designed to have a condensate line draining off into it though. You know the bactera from your waste piping crawling up into your AC coils and breading then getting blown all about the house. The ones I seen had a plug installed in that part. I always thought it was some sort of clean out.
With a fast internet search only thing I can find close to what you have is a Filler -waste and overflow .. .which I would say WOW... that does not meet code. http://www.ukbathrooms.com/shop/bath...ll_system.html