I tried something like this, but instead of a KVM, a remote "terminal" for multiseat in Linux. usbip worked fine, but I couldn't get video to work for a damn. The idea was to have the main computer stream video from one seat to the RP and act as a client for the keyboard and mouse attached to the RP using usbip. Had I been able to get the video portion to work, then next step would have been setting the RP to boot to a fullscreen vlc (or what have you) which displayed the gdm login screen from the main computer.