I had a chance to put together a simple variation of this today.
I looked over the HG_mini Gerber files and found it is possible to use a basic PCB no need to get one made. Doing this I was able to create a HG-Mini for under $20 CD, however I didn't add the sensors or RF.
I wasn't expecting much firing this up but the HG app does see it and you can add X10 devices even without the RF board attached of coarse they don't do any thing, the light and temperature sensor also display as does the GPIO pins, but again nothing usable with out them attached.
What I am a bit disappointed in is no Web UI. there are 2 ports used but the one displays 'Not found: /' (possibly due to nothing sensors or RF connected), the other displays 'This is a Websocket server only!'
I'm not sure what I was expecting from this or if any one would really find this useful. For me the build was a learning experience but I'll flash the firmware with something else.