gravity-fusion/Assets/Editor/BeforeStart.cs

//From https://answers.unity.com/questions/442342/how-to-make-public-variable-read-only-during-run-t.html
using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(BeforeStart))]
public class BeforeStartAttributeDrawer : PropertyDrawer
{
    // Necessary since some properties tend to collapse smaller than their content
    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        return EditorGUI.GetPropertyHeight(property, label, true);
    }

    // Draw a disabled property field
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        GUI.enabled = !Application.isPlaying;
        EditorGUI.PropertyField(position, property, label, true);
        GUI.enabled = true;
    }
}

public class BeforeStart : PropertyAttribute {}