Just out of curiosity, is there any evidence that the Russians are working on such a rocket stage or launch vehicle? Or is this a figment of everyone's imagination?
Pretty much; Angara 5 is about all we'll see for quite a few years. And best case scenario for that would be an upgrade with cryogenic LH2-fueled upper stages at some point. An even better case - but very unlikely - scenario is the A7 concept. But this is just a 'paper rocket' for the forseeable future...
http://spaceflight101.com/spacerockets/angara-a7/
Well, regarding rocket papers, RSC Progess is working on the Soyuz 5 vehicle. It would sport a KbKhA RD-0164 engine with 3.2M, gas-gas injectors and performance quite similar to SpaceX's Raptor. And that engine is in development.
If they can advance with the Soyuz 5 cores, when used as boosters, could power something big. In fact, that's been RSC Progess SHLV project all along.
