The user-experience of a USB ethernet device connecting is much better (I’ve
tried with a real adapter), so if that’s possible it does seem to be worth the
effort.

Right now, no, there is no greybus network driver, sorry, so this isn't
going to work, unless you get the USB host controller code to work,
_AND_ have the correct USB networking driver loaded in the kernel.

That last point you might have a problem with on the system side as
well.