Just to add one more, I got this response from Disney today:
Thank you for contacting the Walt Disney World® Resort.
We are so excited to have you visit for our Mickey's Not So Scary Halloween Party!
Yes, you will be able to make FastPass+® selections for the hours from 4:00pm to 6:30pm, depending on availability. The Party tickets need to be linked to your My Disney Experience® account, and you will need to be within your window of arrival. If you are staying on property, you are able to begin booking your FastPasses® 60 days prior to arrival. If you are not staying with us, you will have a window of 30 days prior to arrival. Once the party starts, FastPasses® are no longer able to be booked, or used. The park is not sold to capacity during the party, so there are almost no wait times for most of the attractions.
It is going to be a great party! Adults are even able to dress up for it. You can come as your favorite character. It is still pretty warm down here, so be sure you plan for that when selecting a costume.
If you have questions or need further assistance, feel free to contact us.
I currently have FP+ scheduled for 4:20-5:20 and 5:20-6:20 that night. I also have 3 FP+ booked for my single day ticket that I will be using 2 days later at Epcot. So far no issues. I'm going to the first party, so I'll report back on how things worked out next month...