First attempt with RequireNodeAttribute

Scripts/Editor/NodeGraphImporter.cs

@@ -0,0 +1,42 @@
+using System;
+using System.IO;
+using System.Linq;
+using UnityEditor;
+using UnityEditor.Experimental.AssetImporters;
+using UnityEngine;
+using XNode;
+
+namespace XNodeEditor {
+    /// <summary> Deals with modified assets </summary>
+    class NodeGraphImporter : AssetPostprocessor {
+        private static void OnPostprocessAllAssets(string[] importedAssets, string[] deletedAssets, string[] movedAssets, string[] movedFromAssetPaths) {
+            foreach (string path in importedAssets) {
+                // Skip processing anything without the .asset extension
+                if (Path.GetExtension(path) != ".asset") continue;
+
+                // Get the object that is requested for deletion
+                NodeGraph graph = AssetDatabase.LoadAssetAtPath<NodeGraph>(path);
+                if (graph == null) continue;
+
+                // Get attributes
+                Type graphType = graph.GetType();
+                NodeGraph.RequireNodeAttribute[] attribs = Array.ConvertAll(
+                    graphType.GetCustomAttributes(typeof(NodeGraph.RequireNodeAttribute), true), x => x as NodeGraph.RequireNodeAttribute);
+
+
+                Vector2 position = Vector2.zero;
+                foreach (NodeGraph.RequireNodeAttribute attrib in attribs) {
+                    if (attrib.type0 != null) {
+                        if (!graph.nodes.Any(x => x.GetType() == attrib.type0)) {
+                            XNode.Node node = graph.AddNode(attrib.type0);
+                            node.position = position;
+                            position.x += 200;
+                            if (node.name == null || node.name.Trim() == "") node.name = NodeEditorUtilities.NodeDefaultName(attrib.type0);
+                            if (!string.IsNullOrEmpty(AssetDatabase.GetAssetPath(graph))) AssetDatabase.AddObjectToAsset(node, graph);
+                        }
+                    }
+                }
+            }
+        }
+    }
+}

Scripts/Editor/NodeGraphImporter.cs.meta

@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 7a816f2790bf3da48a2d6d0035ebc9a0
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Scripts/NodeGraph.cs

@@ -81,5 +81,36 @@ namespace XNode {
             // Remove all nodes prior to graph destruction
             Clear();
         }
+
+#region Attributes
+        /// <summary> Automatically ensures the existance of a certain node type, and prevents it from being deleted. </summary>
+        [AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
+        public class RequireNodeAttribute : Attribute {
+            public Type type0;
+            public Type type1;
+            public Type type2;
+
+            /// <summary> Automatically ensures the existance of a certain node type, and prevents it from being deleted </summary>
+            public RequireNodeAttribute(Type type) {
+                this.type0 = type;
+                this.type1 = null;
+                this.type2 = null;
+            }
+
+            /// <summary> Automatically ensures the existance of a certain node type, and prevents it from being deleted </summary>
+            public RequireNodeAttribute(Type type, Type type2) {
+                this.type0 = type;
+                this.type1 = type2;
+                this.type2 = null;
+            }
+
+            /// <summary> Automatically ensures the existance of a certain node type, and prevents it from being deleted </summary>
+            public RequireNodeAttribute(Type type, Type type2, Type type3) {
+                this.type0 = type;
+                this.type1 = type2;
+                this.type2 = type3;
+            }
+        }
+#endregion
     }
 }