It seems that end should be relatively loose, assuming you didn't jam it in there or use the wrong sized allen (you didn't use an English allen key, did you?) If so, is it possible to shake it out (upside down), or use a high powered magnet? If not, is it possible to use a center punch in the middle of the now-broken allen, allowing you to start a small drill bit? From there, you can either start a screw and pull it, or use larger drill bits until it falls apart (taking care not to damage the expansion bolt).