Sounds like an interesting project for us hardware / software hackers. Probably not for the plug-n-play crowd though.
I do have an earlier design (CM15A2Z) which I deep-sixed when X10 went belly-up. It is a daughterboard that replaces the Cypress MCU in the CM15A and has the same feature set as the Arduino shield design. Users would have to desolder the MCU and EEPROM, replacing them with the daughterboard and 128KB EEPROM. It is certainly not plug-n-play but avoids UL issues.
Now that they are again selling the CM15A,
I may exhume the CM15A2Z design after I get the shield versions released.
After sleeping on this I realized that, since my CM15A2Z daughterboard included an RS232 port (in addition to the USB port), it could support external serial devices like the ESP8266 WiFi module or the HC-05 Bluetooth module. The external devices would need their own 3.3V power supply since the FTDI FT230X USB/SER converter used on the daughterboard cannot supply 3.3V let alone the rather high current needed by the WiFi module..
As there already are interface boards available for both the WiFi and Bluetooth modules, this comes closer to being a Plug'n'Play solution (especially if Authinx would supply the new CM15As with a socketed Cypress MCU). It's likely to appeal to more users than either PLC Shield or mega PLC Shield versions. Authinx is welcome to the design should they like to build them right from the start. A minor tweak of the CM15A PCB to relocate the RF transmitter would allow a larger daughterboard that could directly accommodate the ESP8266 and a 3V3 regulator, thus WiFi enabling the CM15A. (I'll demonstrate this with one of my CM15As.)
I've done a quick edit of the CM15A2Z webpage I created a couple of years back. See...
http://davehouston.org/cm15a2z.htmAnd, since I already have a handful of bare daughterboards, all I need is a few components to build & test this idea. So, stay tuned.
EDIT: On
second (make that third or fourth) thought, I can do this with no changes to the CM15A - although supplying it with a socketed MCU would make this an easy task, even for those with a solder allergy.