I haven't any experience with it, but had been looking at it on the web.
An example uses serial-to-ethernet converters and associates the serial output of each POS terminal (in RS-232, RS-485, or RS-422 formats) to the appropriate analog channel in a DVR, or to an IP camera. The serial overlay can be adjusted for text size and location within the video frame. That footage can subsequently be searched by serial text content. The detailed nature of the text search tools are not made clear in this example (e.g., simple string? Boolean? etc.).
From the example, Hikvision's Serial IO Server appears to be a fairly straightforward piece of commodity gear. For example, we had used a DCB IP6600 serial-to-ethernet router for a similar application. Of course, if the POS terminal serial output is already accessible via ethernet, no serial-to-ethernet converter is necessary.
The critical functionality appears to be within the DVR or network camera and NVR. Note that the hotlinked example calls out specific model numbers with the desired functionality.
If you've already got this far in your research, I apologize that I haven't anything else to contribute. Hopefully other IPVM members who have real experience with this solution can be more helpful.