[NTG-context] Node for startbuffer.

Hello! Given some \startbuffer Blah, blah, blah... \stopbuffer I'd like to get a "node tree" corresponding to how the buffer would be processed at a given point. Using luatex, I'd like to identify certain sub-nodes and change their attributes (to be specific: transparency). Any hint on how to do it? Also, are there any good examples on how to use "nodes"? André Caldas.