There may yet be a light at the end of the tunnel. It took some detective work but I managed to track down the guy who was behind the old Boca Systems who had at one time offered an x-10/ZigBee protocol converter with some logic in the box. I just sent him an Insteon PLM to pay with - he definitely sounded interested in perhaps resurrecting his efforts (no promises, but he’s at least interested in taking a look). Sounded like a retired engineer who knew his stuff and apparently has boards and patents that never made it to market. There may be hope yet. I’ll keep y’all posted on what transpires.
Another option would be to roll-your-own bridge. You could use a ST Shield/Arduino and connect that to a RS232 X-10 Controller. I’m in the process of getting the parts together to try this out. The part to figure out is device management. How do you to let the ST Cloud know about all the devices on the x-10 network.