Autolysis (yeast death and decomposition) is the biggest risk letting your beers rest on the yeast cake. This can produce off flavors, but is usually more of a risk in large conical fermenters. That being said, there are high gravity beers (old ales, barleywines, etc.) that have sat for six months to a year lagering on the yeast cake with no ill effects.
I'm assuming your bock was made using a lager strain, which usually prefer a long cool fermentation for four to six months. I think you'll be fine as long as your samples taste good and you keep everything as sanitary as possible.