> The doors had to remain open and pointing away from the earth
It's the opposite. The orbiter usually kept it doors pointed toward Earth and the "bottom" tiles pointed away from Earth, and orbited with its engines pointed prograde. (Upside-down and backward relative to the atmospheric flight people are familiar with)
Proxy ARP would have the raspberry pi provide its own MAC address as the MAC for the server IP. All the traffic from the clients to the backup server would have to pass through the Pi's network interface for the duration of the session.
With this ARP stand-in, it provides the server's MAC to the client so subsequent traffic to the server doesn't need to pass through the Pi.
Ultimately, it's just a tool, so if the tool needs you to hold it this way and twist, you hold it and twist. And this seems to do the trick. Since it does answer with references for other situations, we needn't concern ourselves with the details.
Use the moment to replace ubuntu-server with Debian and you'll be glad you did when Canonical decides on its next move to ensnare users. Even when I used Ubuntu - back in the early brown-desktop days when they sent out free CD-ROMs to anyone who wanted one - I never felt tempted to use it on a server since it was never clear to me what it offered that Debian could not deliver while it was clear that keeping Debian up to date was (and is) far easier than doing the same with Ubuntu.
Ubuntu had its place in popularising Linux but they jumped the shark a long time ago, now they are just another player jostling for their own niche.