Good Evening,
What you are trying to do is relatively easy to do with an old PC or even a single chip microcontroller. The problem is getting the information into the X-10 control software, that is the difficult part. X-10 is a control system/protocol that is primarily in the digital domain, there are some kludges to take the analog domain into the digital domain but it is not that easy.
In my system the code to receive and transmit the X-10 on the power line is larger then all of the rest of the control code. It runs under DOS and Windows 95. It not only controls the lighting, security it also controls the HVAC system. All of the critical control such as the HVAC is designed fail safe and controlled with the computer. If the computer fails critical units fall back into the original control system and the computer control totally disconnects.
The x10 and RF is monitored with a Merrick unit which needs the baud reset every time power fails. It is running on a AT which has more then enough reserve (code was written in assembler). It has been up since 9-1-2003. Anything important or non fluff is hardwired, it doesn't go intermittent, the toy part is X10.
You can connect a few power flash modules to a heating / cooling thermostat. Most thermostats simply have one or several sets of contacts.
Have Fun
Gil Shultz