eselect bashcomp enable unfortunately seems to lack a
--all option, and I certainly don't want to manually activate each of the 194 entries. Can this be automated somehow, and set up such that packages installed afterwards automatically have their bash completion activated?
I'm afraid the answer is, No.
So while an individual package could potentially support what you want, that wouldn't solve the problem globally.
Your best bet is probably to open a bug at http://bugs.gentoo.org and request the functionality.
While I don't know in detail what this would work like, I should mention that there could be good reason to not globally enable all of them, hundreds of extra entries could potentially cause significant slow down of bash.