In the USA, the CM15A replaced the CM11A (in Europe C12, IIRC) which was RS232. I imagine they thought USB was better as native RS232 ports were disappearing from PCs at about that time. IOW, it's highly unlikely X-10 plans an RS232 version.
There are a couple of solutions.
1. You can add USB host functions to your device using FTDI's Viniculum chip.
2. You can design a daughterboard replacement for the Cypress MCU used in the CM15. I've actually designed two such daughterboards, one with a PIC and one with a ZBasic (AVR) chip. Both had bootloaders so field updates were possible. But I never released them because desoldering the Cypress MCU is not an easy task, even for those experienced with a soldering iron and because it was just too ambitious a project given my age and poor health. (I still plan a daughterboard for the RR501, which I call RR5x5, that adds isolated RS232, isolated 50-ohm BNC antenna connector, and an isolated output port for sending IR/RF with external hardware.)
The first option is the best one for you as it involves no DIY modifications of the CM15 and will work worldwide.
Should you wish to explore the second option, ZBasic worked with me and has included low level X-10 functions in their compiler. They are available in most of their chips although the ZX-32n is probably the most suitable here. The X-10 functions operate in the background more or less like UARTs sending/receiving as needed at ZC. All the user need do is define the pins (some restrictions for ZC) and define the bit arrays for TX & RX. Flags signal various conditions. Optional signals that emulate those in the CM15 are included. You can also control whether signals are active high or active low so you can emulate a TW523/XM10 and interface to legacy controllers.
See my page on the CM15A for oscilloscope screenshots of the various signals...