Share custom compositor nodes among blend files I feel there are some basic nodes currently missing from the node repertoire of the compositor and material editor. One such node (for me at least) is a "vector" node. This would be the 3D equivalent of the 1D "Value" node. Such node would takes 3 inputs X, Y, Z and have one vector output. Why the need for such a node? Some nodes take a vector as input (e.g. "offset" or "scale" etc), but you cannot connect a single value output of a node to the individual values of a 3 valued input. My solution was to create a "vector" group node in which I have XYZ single value inputs and use a "Combine RGBA" node to combine the XYZ values into one 3D (vector) value output. This works fine, however I want to have this custom node available to any of my blend files without having to always use append to grab this custom node any time i need it. 

My question is: Is it possible to make a custom (compositor or material) node that is shared among blend files without explicitly importing that node by appending it from another file? .. perhaps via an add-on script that automatically creates these custom (group) nodes using the python node API?
You could customize your start up file. 
1. Open your start up file
2. Append the node groups you use a lot, click on the F in the node groups and delete the node groups.
3. Go to your default screen
4. CTRL + U save sart up file.
5. Close your file
6. Open a new file, go to the compositor, SHIFT + A --> group --> your node groups