I think you can do it while it's on the cake. Others I'm sure have done it before but if I did that I would definately stretch the fondant out of shape just putting it on the cake even with a rolling pin of sorts. I would go with trying it on the cake.
In regaurds to doing the pattern on BC: How do you get it to not stick? I just bought the impression mat. I am assuming that I just push it gently against the cake and then kinda roll it along the side, pick up, line up the pattern and press again. Is this right? Any tips or hints? I am making a special cake for my mom and I really want it to look great!