I'll continue to let DD trick-or-treat like every other kid and then swap out what she gets for safe treats and a present from the Great Pumpkin.
This is what we do as well. My son trick-or-treats like the other kids and then when we get home it goes directly into the trunk of the car and he gets a cool toy and some safe candy.
Personally I would not register for this, as I would not want to share this information publically.
And on a side note - the candy we give out is safe for dairy, eggs, peanuts & tree nuts. It is not safe for soy or wheat. For all the kids around my area, this works for the other kids with allergies.