rpm -Uvh authenticated proxy access ... just hangs?

For RPM, what you provide to --httpproxy will be forwarded to curl --proxy.

So rpm --httpproxy http://foo:[email protected]:3128.

If you want to troubleshoot, you can edit /usr/lib/rpm/macros and remove --silent in there:

%__urlhelpercmd         /usr/bin/curl
%__urlhelperopts        --silent --show-error --fail --location -o
%__urlhelper_proxyopts   %{?_httpproxy:--proxy %{_httpproxy}%{?_httpport::%{_httpport}}}%{!?_httpproxy:%{nil}}

You can use the usual way of exporting the http_proxy (and/or https_proxy) variable. I believe this will also work with "wget". RPM seems to use "curl" to get the files, so "man curl" may be also useful.

$ rpm -qp http://ftp.jaist.ac.jp/pub/Linux/CentOS/6/os/i386/Packages/centos-release-6-2.el6.centos.7.i686.rpm
curl: (7) Failed to connect to 2001:200:141:feed::feed: Network is unreachable
error: http://ftp.jaist.ac.jp/pub/Linux/CentOS/6/os/i386/Packages/centos-release-6-2.el6.centos.7.i686.rpm: open failed: No such file or directory

$ export http_proxy="http://${proxy_user}:${proxy_pass}@${proxy_server}:${proxy_port}/"
$ rpm -qp http://ftp.jaist.ac.jp/pub/Linux/CentOS/6/os/i386/Packages/centos-release-6-2.el6.centos.7.i686.rpm
centos-release-6-2.el6.centos.7.i686