
I haven't seen any speculation about it. But, the Salyut KB (Design Bureau) 
was planning a new LH/LOX second stage for the Proton which would boost
payload to LEO from about 21000 to 31500 kg. (Geostationary goes from
2600 kg. (Gals launcher version) to 6000 kg.. This scheme was competing
with the Energia-M last year and I haven't heard which won, except now
I recently read that the Central Specialized KB was working on the 
successor to the Soyuz booster which must be the Energia-M. So the early
results are Energia-M won, but this is a guess, nothing is very clear in 
Russia. I'm sure if Salyut KB gets funds from someone they will continue 
their development. 

The Centaur for the Altas is about 3 meters dia. and the Proton 
is 4 so that's a good fit for their existing upper stage, the Block-D
which sets inside a shround just under 4 meters dia. I don't know about
launch loads, etc.. but since the Centaur survives Titan launches which
are probably worse than the Proton (those Titan SRB's probably shake things
up pretty good) it seems feasible. EXCEPT, the Centaur is a very fragile
thing and may require integration on the pad which is not available now.
Protons are assembled and transported horizontially. Does anyone know 
how much stress in the way of a payload a Centaur could support while
bolted to a Proton horizontally and then taken down the rail road track
and erected on the pad?  

They would also need LOX and LH facilities added to the Proton pads 
(unless the new Proton second stage is actually built), and of course
any Centaur support systems and facilities, no doubt imported from the
US at great cost. These systems may viloate US law so there are political
problems to solve in addition to the instabilities in the CIS you mention. 