I am working on tool so you can see what you skin will look like in the game without having to log in. Here is a quick screen shot (using agentfive's MonsterTime skin):
You can select you own background, toggle the main toolbar on and off, and reload you images/XML files on the fly.
This is not an actual interface, so you cannot click buttons and have menus open, but it will give you an idea of what the skin will look like.
Still a ton of work to do before it is finished.
Some of my todo list:
Toggle the tile pattern on/off
Select normal/pressed/rollover mode for buttons
Let me know what you think.