I got a reply from Chris Fischer the same day I asked him about that rumoured sequel to "Of Mice & Mayhem." He says there's no truth to it. He'd like to do another RR story, but he's too busy with his family and career (he's a professional musician) to be working on another huge comic story.
It may be that *someone* else has been talking about doing an unauthorized sequel to "OM&M"... which would be somewhat amusing as "OM&M" is itself unauthorized. But, I can't check into that possibility.