I ended up doing what they mentioned here: Using NSSortDescriptor to keep blank values down the list
I made a BOOL value that indicates whether or not the user has reminder set. So, I sort by presence of a reminder, and then by date.
NSSortDescriptor *sortByDate = [NSSortDescriptor sortDescriptorWithKey:@"reminderDate" ascending:YES];
NSSortDescriptor *sortByHasReminder = [NSSortDescriptor sortDescriptorWithKey:@"hasReminder" ascending:NO];
It works well. As a database enthusiast, I hate having unnecessary fields, but it's the best I can do with NSSFetchedResultsController.