To avoid hacking the source code, the proper way is to [Create Custom Object Drawers].
Please take a look at the [Documentation for Creating Custom Object Drawers], but in short, you simply need to create a class like the following (remember that this is a Unity Editor only functionality, thus to wrap it into an #if UNITY_EDITOR define).
Or, you could move the code from your ActionFlowField method, into the OnGUI method of the ActionFlowFieldDrawer class, if it is only used within the context of NodeCanvas.