I wonder how hard it is to make a fake USB printer.
Like, a printer that takes any printout and goes "yep, that printed just fine" but nothing ever comes out anywhere

@foone almost certainly completely overkill for this task but there's the USB raw gadget samples: https://github.com/xairy/raw-gadget

I've used this to simulate USB devices in the past, and it includes a printer.c example

GitHub - xairy/raw-gadget: USB Raw Gadget β€” a low-level interface for the Linux USB Gadget subsystem

USB Raw Gadget β€” a low-level interface for the Linux USB Gadget subsystem - xairy/raw-gadget

GitHub