Hi shangu,
Great question.
The 3D preview reflects the entire node network, so in order to isolate a single node, you would have to disable all the downstream nodes from the "specific" node.
You can manually deselect nodes by selecting them in the Node Network pane and pressing the "d" key. This may not be the most efficient workflow, and there are certain nodes that may rely on downstream nodes for their final output. For example, the Power Fractal v3 shader has a parameter to "Obey downstream smoothing filters".
It may also be possible to write a script which uses Terrgen RPC to automatically disable nodes downstream from the selected node, and even re-enable them.