I'm afraid this isn't possible yet. Consider making a proposal over here.
Alternatively (if you don't mind the information being displayed in a dock) you could create an EditorPlugin to do what you want. Something like this:
tool
extends EditorPlugin
var dock
func _enter_tree():
dock = preload("res://addons/plugin/plugin.tscn").instance()
add_control_to_dock(DOCK_SLOT_RIGHT_UR, dock)
func _input(event):
var scene_root = get_tree().get_edited_scene_root()
var mouse_coords = scene_root.get_global_mouse_position()
dock.text = str(mouse_coords)
func _exit_tree():
remove_control_from_docks(dock)
dock.free()
where plugin.tscn
is just a scene containg a Label-node, nothing else.