Can you wear a surfing wetsuit to snowboard in for a day under street clothes and a snowboarding jacket as something to keep you dry agile and not have to buy a bunch of boarding gear?

sounds good to me. I surf in temperatures below freezing in 40 degree water and stay toasty warm with a wet suit on. The only problem i can see is it getting to hot (I sweat walking down to the water). I would wear a 4/3 or 3/2 also as a bonus wear your wetsuit gloves. I would also wear something light and water proof as pants like some type of wind breaker material and of course wool socks. This being said i have never tried this so you should probably bring a back up. I do intend to try it this winter though.

DON"T DO IT!

wearing a wetsuit to snowboard in is a bag idea. You'll be very uncomfortable and likely overheat. It's like wearing a sauna suit: There is no where for the sweat to go.

For as cheap snowboard suit get a set of Carharts. It's what the construction guys wear.

Very warm, flexible and almost waterproof.