I thought of a few other ideas, not sure if they would help at all:
1. Perhaps the POS software will work with a CM19A, since AHP could do that in later versions?
2. If you do get a replacement CM15A, but you still want the rest of the system to be Insteon, you might be able to use an X10-to-Insteon bridge (like the EZX10RF, for example) to convert those X10 signals to Insteon. You would still need a CM15A (unless my idea of a CM19A works), but the rest of the system could be Insteon.
3. It sounds like your POS software is using the X10 SDK to send commands to the CM15A. If someone could figure out a way to modify the SDK (I don't know if the source code for it is available or not) to redirect those signals, you might be able to pass them on to an Insteon controller. Again - I'm not a programmer (certainly not a *good* one at any rate), but there are a number of developers here on the forums who might be able to help.