I don't think a tether switch would help you.
There are a couple of easy options for security. One Expensive, one not as expensive.
First, add a motorcycle alarm, like Armed Guard on your snowmobile. I moved my motorcycle/snowmobile pager alarm on my Firecat, but haven't put the battery on it yet. This allows you to wire the sled to send you a page up to 700 feet away if the sled is tampered with. A cable used as a bike chain and switches for "lifting the hood" can be used with this system. The minute it's activated your pager goes off and once your pager goes off, it will continue to alarm or buzz on your belt until you reset it. Once you know someone is messing with your sled, you can get out there and find out what's happening.
Second, buy a bike cable and carry it and a paddlock with you. Lock your sled to your riding buddy's sled. Run the cable through the A-Arms (heck you can even park the sleds facing in opposite directions for additional security parked spindle to spindle). Most thefts are "joyride" related or pickups where a sled is picked up and thrown into a pickup truck. If you have your sled chained to a pals, the thief will probably move on to another sled or perhaps pick up a different sled as two don't lift very easily.
The other thing you may want to do is park away from easy access by motor vehicles if you stop by a local restraunt or bar (to eat lunch). It's easier to walk another 20 yards and have the sled away from a theiving pickup truck.
I'm sure there are other ways, but these are the easiest.
Greg