I FOUND IT!!!! The hunt is over for me this season. I have found a McDonald's that actually sells the infamously hard-to-find eggnog milkshake that I wrote about last year. Coincidentally, the McDonald's I found the shake at is in Lafayette, the same town that the treehouse house might be built in.
Please feel free to post a comment if/where you've found the eggnog milkshake too!