We were at WDW for NYE two years ago. We wanted to leave Epcot during the afternoon and return for our PS later that night. Before we exited the park, I had a 5 minute conversation with the security people and the gate attendants. Before we left I wanted to understand whether we would get back in.
They explained to me that there are different levels of restricting entry into the park based upon how full the park is. Because we were staying at BWV we would be the last group of people excluded from Epcot. If your resort is associated with a given park then you are the last to be shut out. Hence, Epcot & MGM would be associated with BWV, BWI, YC/BC, BCV. The MK would be associated with the Contemporary, Poly, WL, WLV, and GF.
They stated that Priority Seatings don't factor into their decision.
It was about 3:00 pm on NYE day and I remember they told us that MK was completely closed to any new entries and that MGM and Epcot were both turning away non-Disney Resort guests (i.e. if you weren't staying on property the only park open to you was AK). I don't know if later in the day the parks changed to a different restriction level.
Sorry, probably not what you wanted to hear. Maybe someone else has had a different experience.
Troy