Connecting a light-switch to turn thermostat on/off? (re: basement heat)
Yes, that should be very straightforward.  Simply wire the switch to interrupt the "W" wire, which is the one the thermostat uses to ask for heat.  Its request will be unanswered obviously.  Do this between the thermostat and what I presume is a big relay with a 24v coil that switches the 240v power on to the heaters.  Turn the 240v off at the breakers before touching the 24v signal line, in the off chance the transformer or relay has a 240v-24v fault of some kind.  

As long as it's an old fashioned dumb thermostat, that'll be fine.  Now, if you ever fit a Nest or other smart thermostat, different story.  It configures by sensing if a relay coil is connected to each wire. It will not like a relay that disappears entirely.