I had something much like this in mind and agree that is a great idea. Both the CGI::Application and Catalyst frameworks have some dispatching built in, allowing you to use a single script for that. The CGI::Application is currently revamping CGI::Application::Dispatch to be even more powerful in this regard.
I'm not sure what you mean by the "alternative interface" to CPAN, because that's the core of what the idea is. It provides an interface to CPAN that generates tarballs which are easy to unpack and upload without shell access.
For fancy configurations, a web-based installation wizard might well talk to CPAN directly to install additional stuff.
