I don't think milk is good for them and I think I would be reserved about feeding baking soda and powder, but the rest seems legit.
Then again, like humans and dogs, horses can tollerate small amounts of harmful foods. Lots of trees and plants are poisinous to horses but they can eat a few leaves without keeling over (thank the lord!). My old dog was a rescue, lived to 17 and had a very varied diet including no-nos like grapes and chocolate. Obviously we (myself and my parents) werent feeding these in huge amounts, but my point is animals are pretty good at not dropping dead because they ate a small amount of something that's technically a toxin.
I do know people who have simply mixed up some oats, syrup and a bit of flour (i think) to make simple flapjack style things to share with the ponies. Keeping it simple can be just as tasty!!
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz