Good question. I did a quick search of the configuration apis and the CTI protocol and didn't seem to be able to find how to get the UCCX version via API. There are a bunch of messages in the CTI protocol, so maybe I missed something. I know this is very backwards, but you probably already know you can get this via CLI. Maybe you can automate it?
The problem with the CLI is its complex and slow, it typically takes a minute just to login.
I was always a little surprised the version was not in the SystemInfo page
I totally agree with the slowness of the CLI, which is why I said it is a little backwards.
I glanced through the documentation, but maybe you can take a second pass through the different CCX APIs here: https://developer.cisco.com/docs/contact-center-express/