How will you know if people tend to pick the same types of events, and, therefore, create big non-random clusters of choices that might be easily brute-forced?
Taking an idea from https://gist.github.com/3840286...
.... you could store a small number of bitcoin at private key = SHA256(memory_key), store the bulk of bitcoin at scrypt(Name+PIN+memory_key), and tell users to choose a new memory key if the SHA256(memory_key) coins are either ever spent or if that key ever gets funds from somebody else.
Because that means somebody else chose the same memory key.