Are there any online recursive filters that can approximate local, time-varying minimum and maximum values of a time series?
I think you are looking for a "peak" detectors. They are not filters in the conventional sense. Typically the output is increased when the incoming sample is either above a certain threshold or above the current output and decreased if it's smaller. The increase and decrease are often just simple exponential with time constants determined by your application. The increase time constant is typically called "attack" time and the decrease time constant is "decay" time.