I noticed that I wasn't using my pacnew scripts for these files so I went ahead and wrote yet another script.
merge-passwd will let you interactively merge multiple passwd-like files (basically anything that follows the line format "foo:bar:baz:etc", such as /etc/passwd, /etc/group).
It collects all entries from all files. Entries that are the same are kept automatically. The user will be prompted to select those that differ, including deletions. By default it will print the merged file to stdout but it can be written directly to a file. I recommend using the "--confirm" option when overwriting system files.
I may add this to the pacnew_scripts package or its own package depending on feedback.