The best way to do this is using communicating thermostats - you can adapt the HAI or RCS ones to control just about any type of heat using the appropriate relay. Of course, this would be a bit costly because you have to buy 8 of the communicating thermostats and get a Cat5 or so to each thermostat - then you can daisy chain them at the Elk (I think 4 HAI's to one XSP IIRC?).
The reason this is better is that you still have thermostats at all time - and instead of cutting power to your baseboard heaters, you have the ability to set specific temperatures or setbacks. You can program a task to set them all back or restore the temp very easily - but still also have your freeze protection by having a minimum allowed temp.
The alternative which I think people are eluding to is to just use a contactor/relay to hard-cut power to the heaters so they don't work - via the M1. While this technically works, it's a bit more of a kludge, and you don't really have the freeze protection unless you build it in with thermostats... all running through the M1. I like my M1, but I don't want to rely on a single system quite that much.
One other potential option - a lot of decent thermostats do have an interface switch basically - like a low-voltage contact you could hook into a simple relay off the M1 - that when switched, it puts the thermostat in "Setback" mode - and when switched again, restores the temps. You set the setback temperatures in the thermostat - and the system is always powered to a minimum temp.
I don't know about you, but I don't let my house drop below 66 degrees. At a point, the rapidly fluctuating temperatures can wreak havoc on a house.