How do you set up the interface "back" to the host?

Is it something like:

USB directly to the guest OS, then an emulated serial port in the guest OS back that the host OS connects to?

right, I have a hub that I connect it to because (shocker) my computer lacks any usb-a ports, so I just pass through the hub, and the serial children it contains are passed through. I just wish the same could be the case for firewire, I'm not willing to pony up the cash to make the dongle from hell (fw400-fw800-tb2-tb3) that may allow thunderbolt passthrough(?)

the prior art in this area is going to be PCI&USB passthrough implemented in qemu and xen, with related but separate in-guest or 'in-host' virtual devices representing the bridged device.

Some related work in the SR-IOV & iommu space makes this a lot easier to implement as well. I would be very surprised if zero new security edge cases get discovered in the next five years or so however. Regardless, I'd look forward to seeing the results of RedoxOS's work here, as this would be a practical alternate implementation of driver domains like you see used in Xen and Qubes.

I agree, as we get even further away from the 90s and 2000s and data gets more valuable, having mechanisms like this to access old systems is vital now that it's getting harder to make "missing link" type machines that have parallel/scsi/ATA/FW/etc. to dump old data