As Tyggress mentioned, EMH is only available for on-site guests and it primarily impacts the lines for RSR and TSMM at park opening. They do not start distributing FPs until the park opens for everyone. Based on the fact you mentioned going to a Thursday MHP, I'm guessing you're asking about Friday, October 23. I haven't heard of any conventions or events that day that would increase DCA crowds above what would be expected on a Friday during HalloweenTime.
RSR have generally been lasting longer lately (since Soarin' reopened), but I would strongly think about getting the FP before going to get the Anna makeover at 10:00 am. By then the park will have been distributing FPs for two hours, so you're likely to get a return time for late afternoon or evening. They do seem to be lasting for several hours these days, but I would still aim to get one within the first 1 to 2 hours of the day to be safe. Olaf doesn't use the FP/pager system anymore, so it's all stand-by. Wait times haven't been to bad, but it can vary. A&E use a FP/Return Ticket system, but those don't seem to be running out terribly early. Given that the A&E FPs are distributed near the park entrance (on Hollywood Blvd), I would stop by to see what the return time is and decide if you can make it to your makeover and back in time.
Regardless of the night, DCA always seems to see an attendance bump on nights when there is a MHP. Guests that do not attend the party and that have park hoppers seem to head over to DCA. On Fridays, DCA is open late (midnight), so I would expect to see quite an attendance jump around 7:00 pm when MHP starts.
