Editor\InspectorDisabledAttributeEditor.cs

using UnityEditor;
using UnityEngine;

namespace FullInspector.Modules.Attributes {
    [CustomAttributePropertyEditor(typeof(InspectorDisabledAttribute), ReplaceOthers = true)]
    public class InspectorDisabledAttributeEditor<T> : AttributePropertyEditor<T, InspectorDisabledAttribute> {
        protected override T Edit(Rect region, GUIContent label, T element, InspectorDisabledAttribute attribute) {
            PropertyEditorChain chain = PropertyEditor.Get(typeof(T), null);

            EditorGUI.BeginDisabledGroup(true);
            element = chain.FirstEditor.Edit(region, label, element);
            EditorGUI.EndDisabledGroup();

            return element;
        }

        protected override float GetElementHeight(GUIContent label, T element, InspectorDisabledAttribute attribute) {
            PropertyEditorChain chain = PropertyEditor.Get(typeof(T), null);
            return chain.FirstEditor.GetElementHeight(label, element);
        }
    }
}

InspectorDisabledAttribute.cs

using System;
using UnityEngine;

namespace FullInspector {
    /// <summary>
    /// Draws the regular property editor but with a disabled GUI. With the current implementation
    /// this is not compatible with other attribute editors.
    /// </summary>
    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
    public class InspectorDisabledAttribute : Attribute {
    }
}