We have not experienced it personally, but last year there were lots of Teal stops for allergy-friendly "treats" (and I think not all of the "treats" were food related). I saw kids with Teal bags as well.
There is so much to do at the party besides trick-or-treating; the Boo To You parade, the stage show, and so many characters to meet! You can search for Hidden Mickeys (give her a camera and make a scrapbook when you get home to remember the Big Hunt!) make pressed pennies and pin trading is great fun as well!
If I were in your shoes, I might try to bring along some non-food treats, and slip them to CMs quickly (maybe in a little treat bag that is sealed) so that the CM can give your little Princess her special treat. Or, maybe set a timer on your phone and every 30 minutes, it's "treat time!" and you pull out another little non-food treat!
I hope you all have a wonderful time -
MNSSHP is one of our favorite events!