It seems every time I find a boatload of new articles, I find another person who played Rush at some point or the other. That's fine, but sometimes (such as this case) we are only told, 'Hey this guy played Rush when Rush wasn't there'. Oh, now when would that be? When he left the series (uh, no one played Rush after he left the series, did they? Maybe they are talking about Series 3... let's not go there).
This article mentions Jack Chalbeck. It's very possible (maybe even likely) that Chalbeck played one of Rush's friends during the Dottie Brainfeeble era.
But who knows?