Oh, it will definitly happen. I can give get some MOS (yea right) or PSQM scores in the above mentioned scenario. I have done g.711 to PSTN to G.711 to PSTN and PSTN to G.711 to PSTN and PSTN to G.711 to PSNT to G.711.
What I can not do is take some sort of standardized sample, go from G.711 to G.723.1M or G.729a and save the resulting sample.
Anyone know of a lab or testing site for this kind of modeling?