As mentioned by others, there are USB-C docking stations/monitors where a single cable provides power to the device, takes video from the device, and sends keyboard/mouse events to the device. I should have asked whether software on a laptop could emulate a USB-C docking station, i.e. DP display sink + keyboard + mouse, since docking stations already exist.
> Communications between such a thingamajig capture card and a laptop has nothing to do with DP Alt mode or USB HID, it's whatever custom USB packet types that capture card manufacturer comes up with.
Revisiting the question above, could a manufacturer make capture card output compatible with existing USB-C docking stations, instead of inventing bespoke USB packet types? If custom hardware is needed, why not emulate standard protocols?
I was surprised to learn that Windows/Linux/Mac/Android userspace software can encode/decode custom USB packets from a USB-C cable, without a custom kernel driver. Could RP2350 implement a similar custom protocol at the other end of the cable, removing the need for a hardware capture device?