Thanks for the suggestions and sharing results. I think I'll go with USB network device solution. I've tried the sample "samples/net/echo_server" and, with the addition of the overlay-netusb(?) to the prj.config, it came up and worked. The current docs have the 3 additional linux side commands to add the enumerated net device to the linux ip stack, bring it up and, lastly, provide a route. Adding the overlay config file in the cmake is a nice trick, but took a bit of digging to find an example. I hope this solution will be stable...will report on results. It seems to have fewer moving (and, perhaps, simpler) parts on the linux side than using slip.
This issue has come up because my app needs to stream data from a ble (hopefully bluetooth 5) net into a linux platform. Initially tried setting up a HCI connection to zephyr hci firmware running in the ble dongle; but, couldn't get the HCI interface to come up on the linux side. While wading through the btattach and config docs, I came across the usb network device option. Fortunately, I have sufficient resources on the dongle to run a dual stack (ip/ble) and add an app to bring the stream into the linux box as ip - as opposed to hci + ble/ipv6. At least that's my plan at the moment. But, I am curious about how others are dealing with this issue....are folks using (or waiting for!) commercial ble/ip gateways to bridge this gap or is there another solution I'm not aware of yet?