Can't say that I've seen one of these up close, but I asked around on some other forums. Not sure if you're a member of the Yahoo singles forum...you might get some good advice over there.
What I heard was that, yes, it can be done without removing the engine. Someone specifically said you'd need a 22mm openend wrench that is 4mm thick. Also, depending on where you get the parts (Blue Moon?), you might ask them for advice on how to proceed.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
I don't quite see how to shift the mounting block enough to get the new rubber mount started.