Sounds good to me, but I would most likely require a str check to see if the character can actually hang onto the wall once they make contact, spider climb doesn't automatically make you immune to being subjected to a force other then the force you exert. Course this is just what I would do. On second thought I would prolly only do this if the horse was moving faster then their normal movement rate. Anything double+ movement rate would require a str check, the DC would be dependent on how fast the horse is moving. Ever see a car run into a cliff face at 10mph? It isn't pretty. =op But that is my own ruling on it if I was a DM, don't really see anything particulary wrong or unbalancing with not having to make a str check to do it.