To change the signpost's windowskin at all, go to PokemonField and find class LocationWindow. In the def initialize there, put the line @window.setSkin("Graphics/Windowskins/goldskin"). The particular windowskin you use may vary.
From there, it's a matter of deciding how you want to change the windowskin for each different map. It's simple enough to do, so long as you've got a system sorted out.