Emerge should have an option like "--config-only" to re-emerge/restore the original configuration of a program (i.e. the files in /etc). In combination with CONFIG_PROTECT this is a good option to test several configurations of a program without having to save the original configfiles. Additional in case of misconfiguration (imagine you use a new program where you don't know how to config it) you can easily restore config-files without re-emerging the whole bunch. Reproducible: Always Steps to Reproduce: 1. 2. 3.
This would take a lot of effort for something that is already available by alternative means. You can add "buildpkg" to your FEATURES to always have binary packages built which will allow for fast reinstallations. Doing it specifically for config files would mean an extension to ebuild functionality or a whole lot of hacking to installation support plus a new type of repository to keep these files in.