Well, how extremely embarrassing
that it was that simple, thanks a lot for opening my eyes... Although I have to say I was slightly misled by
* Adding module to moduledb.
Switching to nvidia OpenGL interface... done
produced by the ebuild every time you install/upgrade the drivers, and by the fact that I don't remember ever having to do that on my other three Gentoo/nVidia systems (but it was certainly a while ago since I installed them from scratch...). Still, I've got a few io-issues with the system, but that is for another forum...
Once again, thanks Adamk75 for pointing me to what should've been obvious!