Hello Stu, can you please give me more information about those limitations you are encountering. Meaning, if you are running into issues or otherwise has problems connecting to UPM, we would very much like to know what they are.
UPM currently has no CLI, but they could manually curl the proper endpoint (curl https://packages.unity.com/ ) then sift through the output to get the tarball download link, retrieve it using curl (curl --output -.tgz) and then add it as a local dependency.