Hello and sorry for the late reply.
Hmm, regarding the ObservableInput, probably the best way right now, would be to make the FlowNode class a “partial” class and as such, you will be able to provide extra registration methods (or other functionality as well) without changing the source code itself. If making FlowNode a partial class works for your use case, I will gladly make it partial officially for the next version.

Furthermore and based on your use case, I am thinking that maybe an OnSceneGUI callback could be added for nodes, so that editor specific utilities like Handles.PositionHandle can be used much easier.

Let me know what you think on the above.
Thank you.