A thermostat really triggers a relay, couldnt you feed the thermostat to an input for the elk and then link to the heating system from there? This way the elk has an understanding of what the thermostat is requesting, and it can also apply some control to the heat.
It is clearly not as good as hooking it to a serial expander, but it might be an option for the short term.
A good question might be to ask how they want to use it with the elk, as that might give more options (they may not be looking for something real complicated).
Maybe I'm oversimplifying, but maybe this gives you an option.