To be honest, in all my years I have never seen Mickey Mouse doing meet & greets in front of the castle. If he appears early in the morning anywhere on Main Street it’s usually closer to the entrance of Main Street, right after you pass under the train tunnels. That’s been my experience anyway. The only photos I have ever seen of guests with Mickey in front of the castle has either been press or promotional photos.
Although, as part of the new “What Will You Celebrate” promo, I believe there is an option that allows you to actually hire a professional Disney photographer to follow your family around and take great shots. I also know you can ‘hire’ a character to appear for about $800 for 30 minutes but I think that’s for private appearances away from the public.
Unfortunately, I think you’re either going to have to get a great photo program to merge two photos or settle for meeting the big guy in official meet & greet locations.
I could be wrong, maybe someone like RowdyRaider or Vlogilvie would know better!