I think Fluttershy is the perfect choice to play Alice, especially since her pet bunny, Angel, makes a great White Rabbit.
And who else to play the Mad Hatter but Pinkie Pie?
I imagine the conversation would go something like this:
Pinkie: "Why is a raven like a writing desk?"
Fluttershy: "Umm.... I dunno. But I know a lot about ravens, so maybe I can figure this out..."
Pinkie: "What are you going on about?"
Fluttershy: "Err... Didn't you just ask me a riddle?"
Pinkie: "Riddle? Who said anything about a riddle?"
Fluttershy: ".... Um. You."
Pinkie: "You? There's a letter "U" in CUPCAKES!!!"
MLP FiM is copyrighted by Hasbro and produced by