The old releases archive of Ubuntu does not have an rsync server, but I am using a wget command on our server that only fetches files if the ones on their server are newer than ours. And since Zesty is frozen in old releases, it should never use bandwidth on our server beyond the first fetching of the Packages files when we run our routine to get the debinx data. Otherwise, wgetting the entire Packages.xz each time we fetch debinx data would not be acceptable, especially for a deprecated release.
On the user's end, there are two files that need to exist with the listed contents to make use of the Ubuntu old releases archive:
/opt/debmirror:
http://old-releases.ubuntu.com/ubuntu
/opt/securitymirror:
http://old-releases.ubuntu.com/ubuntu
The two above files should be created and backed up. Then dCore-zesty users can install packages from the Ubuntu Zesty archive. The prebuilt section is available with no adjustments needed.
Thanks for the idea. I will do this for dCore-zesty, and perhaps others in the future.