I have seen many examples of folks who have actually taken a cake to a cheese grater and spread the resulting bits over grain to some sucesss. of course if the cakes have been contamiinated, things get harder. one could also shave the outside of the cake and take cores wth which to propagate.
Expert? depends on what that means. I figure 40 years might give me somewhat of an edge.
It's very difficult to keep the BRF cake sterile while transferring to the grain. A lot of people have tried it on the shroomery and resulted in failure. Not to mention most people nowadays use a dry verm layer on the top as a filter. How are you going to remove the cake without disturbing the verm layer that holds millions if not trillions of mold spores?