Just a quick note: There are single KNX/EIB installations with tens of thousands of installed nodes. A total of a million nodes is nothing.
And feel free to provide a driver, we will incorporate it. LinuxMCE is capable of supporting anything, someone just has to write drivers for it.
At the moment, there is no one writing a driver for Zigbee, that I know of. Most development effort is currently in the area of Z-Wave and KNX/EIB.