Bacterius, thanks for such a quick answer. Meanwhile, I came across something called the Rule of Three (Hanley 1983), which was originally applied to estimate probabilities of failures in medical procedures.

It says that "if none of n patients showed the event about which we are concerned, we can be 95% confident that the chance of this event is at most 3 in n (i.e. 3/n). In other words, the upper 95% confidence limit of a 0/n rate is approximately 3/n."

http://www.ncbi.nlm.nih.gov/pmc/arti...00608-0045.pdf
That seems like one possible approach, right?