I have not used a SoftCard with CP/M in many years, nor have I used Zterm. However, I believe the software does not need to interface with the serial card directly, that is one of the jobs of CP/M. In other words, as long as your version of CP/M supports the serial card, your application will as well unless the developers choose to work with the hardware directly.
Again, it has been years, but I had the same issue using a SuperSerial and CP/M. I updated to a newer version of CP/M and it began working. I think I was using versions 2.2 and updated to 2.23 and placed the SuperSerial in slot 3.
Hope this helps,
Marvin