Here's the change I brought, yet nothing happens:
[CustomActionEditor(typeof(DebugFloat))]
[ActionCategory("LOLIPOP")] // added that line
public class CustomActionEditorTest : CustomActionEditor
It looks like only specific commands of the type "GUILayout" can alter anything, and only inside the actions. There's no higher level influence on how an action will be ordered in the browser.
Or there could, but then I'd need to find the special secret command that is exclusive to Playmaker. I don't think it exists.
[ActionCategory()] might only work on classes that inherit from FsmStateAction.