Fantastic map from a GPS I gather? Folks have a hard time finding depth charts for Magog. If there could be numbers on the declination lines..... Yeah, I have to say that from what I know from the 30 + years hard water fishing there, that ATV's are ok. Again I'd stress having it registered. Many times my dad would drive all the way up from his house in Brattleboro, (we have a family farm in Coventry) with his ATV on the trailer. We would off load right on the marsh side of the road, hook up the wheeler to the trailer, load up and head out! I know we ran into game wardens on some occasions.
Unless something has really changed in the past few years, I'd say you are set. Also, there isn't any kind of the Kiosk, or F&W board right there where you access the water.
Last year my buddy brought out his Honda side by side when the snow was deep. At the end of the day I was back at the truck loading up when he came around the corner with the rest of the guys in our party. Going like a bat out of hell just so he didn't get stuck, they hit a rut which spun them sideways and shot all four of them out of the rig like they were in some kind of weird circus stunt. They were about 200 yds from shore and by the time I could reach them they were all back on their feet laughing it off. It could have ended differently.
Snow machines and Wheelers everywhere when I fished on the 29th. I was deep then, deeper now! Good luck!!