It is now possible to mark a master node as active in a shader graph (but not in others like sub graphs or remap graphs)

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/DrawData/RemapMasterNodeDrawData.cs

     }
 
     [Serializable]
-    public class RemapMasterNodeDrawData : MaterialNodeDrawData
+    public class RemapMasterNodeDrawData : MasterNodeDrawData
-            return new List<GraphElementData> { instance };
+            var controls = new List<GraphElementData>(base.GetControlData());
+            controls.Add(instance);
+            return controls;
         }
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/DrawData/SurfaceMasterDrawData.cs

     }
 
     [Serializable]
-    public class SurfaceMasterDrawData : MaterialNodeDrawData
+    public class SurfaceMasterDrawData : MasterNodeDrawData
-            return new List<GraphElementData> { instance };
+            var controls = new List<GraphElementData>(base.GetControlData());
+            controls.Add(instance);
+            return controls;
         }
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Runtime/Graphs/MaterialGraph.cs

 using System;
 using System.Linq;
+using UnityEngine.Graphing;
 
 namespace UnityEngine.MaterialGraph
 {
                     return found;
 
                 return GetNodes<IMasterNode>().FirstOrDefault();
+            }
+            set
+            {
+                var previousMaster = masterNode;
+
+                if (value == null)
+                    m_ActiveMasterNodeGUID = Guid.NewGuid();
+                else
+                    m_ActiveMasterNodeGUID = value.guid;
+                
+                masterNode.onModified(masterNode, ModificationScope.Node);
+                previousMaster.onModified(previousMaster, ModificationScope.Node);
             }
         }
 

MaterialGraphProject/Assets/UnityShaderEditor/Runtime/Nodes/AbstractMasterNode.cs

             get { return false; }
         }
 
+        public bool isActiveMaster
+        {
+            get
+            {
+                var graph = owner as MaterialGraph;
+                return graph != null && graph.masterNode == this;
+            }
+            set
+            {
+                var graph = owner as MaterialGraph;
+                if (graph == null || value == isActiveMaster)
+                    return;
+                graph.masterNode = value ? this : null;
+            }
+        }
+
+        public bool canBeActiveMaster
+        {
+            get { return owner is MaterialGraph; }
+        }
+
         public abstract string GetFullShader(GenerationMode mode, out List<PropertyGenerator.TextureInfo> configuredTextures);
         public abstract string GetSubShader(GenerationMode mode, PropertyGenerator shaderPropertiesVisitor);
     }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/DrawData/MasterNodeDrawData.cs

+using System;
+using System.Collections.Generic;
+using RMGUI.GraphView;
+using UnityEditor.Graphing.Drawing;
+using UnityEngine.MaterialGraph;
+using UnityEngine;
+
+namespace UnityEditor.MaterialGraph.Drawing
+{
+    class MasterNodeControlDrawData : ControlDrawData
+    {
+        public override void OnGUIHandler()
+        {
+            base.OnGUIHandler();
+
+            var masterNode = node as AbstractMasterNode;
+            if (masterNode == null || !masterNode.canBeActiveMaster)
+                return;
+            
+            using (new EditorGUI.DisabledScope(masterNode.isActiveMaster))
+                masterNode.isActiveMaster |= GUILayout.Button("Set active");
+        }
+
+        public override float GetHeight()
+        {
+            var masterNode = node as AbstractMasterNode;
+            if (masterNode == null || !masterNode.canBeActiveMaster)
+                return 0f;
+            return EditorGUIUtility.singleLineHeight + 3 * EditorGUIUtility.standardVerticalSpacing;
+        }
+    }
+
+    [Serializable]
+    public class MasterNodeDrawData : MaterialNodeDrawData
+    {
+        protected override IEnumerable<GraphElementData> GetControlData()
+        {
+            var instance = CreateInstance<MasterNodeControlDrawData>();
+            instance.Initialize(node);
+            return new List<GraphElementData> { instance };
+        }
+    }
+}

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/DrawData/MasterNodeDrawData.cs.meta

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