It could be a serial interface to connect a touchscreen to the cd-i player, so costumers can use the screen as a input device. That seems like the most viable option. At least it is something that uses an EPROM and draws power from the RF modulator power source. It's remarkable that the disc is copyrighted in the year 2000! That's when CD-i was long over, but apparently they still saw a market to use CD-i as retail units. Maybe they were cheap as DVS otherwise wasn't able to sell them anymore: In the consumer market the CD-i sales were near 0 after the demise in 1997.
If it would indeed be an interface for a touchscreen, perhaps it was something like this Videotronic unit. I'm still wondering how the touchscreen is actually connected and I think it is not as simple as the same connector as other pointing devices on CD-i
Update: I'm reading through old Philips Media brochures that are describing a touch screen as one of the wired pointing devices. Still unclear to me how that would work, does the Videotronic have an output connector that you could plug into the input port of a CD-i player?
[Thanks, Dale Mangrum, Patrick Selten, Horst Rosenbaum]