i create TAGS files for sources installed under /usr/src/debug/<category>/<P> and include them in TAGS files for my projects. portage would be nicer if it could create TAGS on a per-package basis when using FEATURE installsources. many projects define a make targets for tags. portage users should be able to specify a make.conf TAGS variable as ctags, etags, gtags, or other tagging systems. portage could have a fall-back mechanism for creating tags using any of the tagging systems.
I think it would be best to make this some type of add-on rather than have portage be directly aware of it. As it is, you can define a post_src_install function (user phase hook) in /etc/portage/bashrc and generate the tags there.
Agreed, beyond portages scope.