Currently the webDAV plug-in doesn't have an event API you could hook into for Open or Save events. The backend classes don't provide events. It works the same as when you open your file directly using ..nsf/view/docid/$File/Filename.txt -> this also doesn't trigger an event.
Having said that, one could implement a subscription hook so the servlet would call an external class on such an event. The challenge with aiming for inside the NSF: the class would not necessarily be loaded. Even if you put it into an applicationContext and preload the XPages app, it might time out.
The recommended way would to implement 2 classes in the plug-in that do the specifics you are looking for.
What are you trying to do?