Since this is you (gerrtunk) asking, I'm not quite sure if I got you wrong, because the solution to what I have in mind is quite simple and a pretty basic thing... so if I got you wrong, don't mind me ^^"
Because of the generic way RM scenes are built - if you stick to the standard - windows are in their nature independent from scenes entirely. If you take Window_Gold as an example: It's an instance of Window_Base, getting defined for itself, however is used in Scene_Menu, Scene_Map, Scene_Shop and who knows in how many of the custom scripts without any modification needed to the actual window itself. It just displays the money, whereever you want.
You can do exactly the same by defining a Window_Bazinga as an instance of Window_Base, then simply reference it as on object in the scene you want (i.e. @bazinga_window = Window_Bazinga.new).
If you need customization, you can always use arguments to modify the window within it's own class. For example, you could do this initialization method:
class Window_Bazinga < Window_Base
def initialize(width = 600)
@width = width
# ...
end
end
That would default to a width of 600 if no parameters are supplied, and would adjust to the given width if there are.
Assuming you knew all this really... what were you talking about? ^^
By the way: Game_Map has nothing to do with it at all.