Edit:

I think there's a mixup in shader errors here, THIS is the error im getting for this shader:

NullReferenceException: Object reference not set to an instance of an object

ShaderForge.SF_Node.GetInputData (System.String id)

ShaderForge.SFN_Panner.IsUniformOutput ()

ShaderForge.SF_NodePreview.Combine ()

ShaderForge.SF_Node.RefreshValue (Int32 ia, Int32 ib)

ShaderForge.SFN_Panner.OnUpdateNode (NodeUpdateType updType, Boolean cascade)

ShaderForge.SF_NodeConnection.Disconnect (Boolean force, Boolean callback)

ShaderForge.SF_NodeConnection.LinkTo (ShaderForge.SF_NodeConnection other, LinkingMethod linkMethod)

ShaderForge.SF_EditorNodeView.ReconnectConnectedPending ()

ShaderForge.SF_Parser.LoadFromNodeData (System.String data)

ShaderForge.SF_Parser.ParseNodeDataFromShader (ShaderForge.SF_Editor editor, UnityEngine.Shader s)

ShaderForge.SF_Editor.InitializeInstance (UnityEngine.Shader initShader)

ShaderForge.SF_Editor.Init (UnityEngine.Shader initShader)

UnityEditor.ShaderForgeInspector.OnInspectorGUI ()

UnityEditor.InspectorWindow.DrawEditors (Boolean isRepaintEvent, UnityEditor.Editor[] editors, Boolean eyeDropperDirty) (at C:/BuildAgent/work/d3d49558e4d408f4/Editor/Mono/Inspector/InspectorWindow.cs:850)

UnityEditor.DockArea:OnGUI()

Quick Question: Is the refract shader also interpolated per-vertex? because i'm getting a lot of the similar issues with refraction also:

I agree a generalised system is hard to do, especially when you have custom blend modes available.

Perhaps when it comes to custom blend modes, specifying a custom fog colour could be an option? so for example, for a modulated x2 blend mode I could specify a fog colour of 127,127,127, or for multiplicative, 255,255,255 etc?

There are similar issues with refractive shaders also

It looks like it's not blending additive materials with fog properly