At any rate, I figured out where to fix the unfeature: A file '_tilde' in path/to//zsh/4.3.4/functions/Completion/Zsh. It happens to be /usr/share/zsh/4.3.4/functions/Completion/Zsh on my ubuntu workstation.
... around line 22 ... while _tags; do _requested users && _users "$suf[@]" "$@" && ret=0 _requested named-directories expl 'named directory' \ ...Comment out the bold line above (starting '_requested users ...') and restart zsh. This only disabled the username completion part of zsh's cd command, but that's good enough for now. If I ever accidentally typo and cd to someone's home directory, I can just popd to return to my previous location.