[..]
Hmm, that begs the question: why run the instances yourself in the first place? Why not simply ask a bunch of clients about their opinion about the latest block (height and hash or whatever) via the protocol itself? Would that also be workable or does that approach have a problem?
could be something like this be implemented on the client to measure a potential fork?
probably. I don't know enough about the protocol myself to answer this question. But say you have 8 connections and one reports a different hash for the newest block (or a lower chain height)... this wouldn't in itself mean much. You'd have to have access to a larger sample, so maybe it'd make sense to implement this as a service with an api that publishes some kind of "alert level" to be used by merchants to halt accepting bitcoin above a certain treshold.