first of all, this is a great piece of artwork you did there! about the editor - currently we don't have such an editor (well, there is one, but it isn't working because many things changed in the mediaportal skinning engine lately)
if you've got enough time you can edit the xml files located in "Mediaportal/Skin/SomeSkin/*.xml"
the best way to start your first skin is to copy all the content over from the "BlueTwo" skin and edit that conent...
if you got problems [in case you want to spend all the time] you are welcome to post here again and we are willing to help you further on.
the problem is that creating skins for mediaportal isn't documented somewhere, so it's a quite hard start...
nevertheless to bring up good news - it is planned to use XAML for the mediaportal skin engine in the future, and that's where you might come in better as you seem to have some WinFX experiance. i'm sorry that i'm not able to tell you when that future will begin, but you're also welcome to help us developing the new skin engine (people with xaml / winfx knowledge are always welcome)
if you got any interest in helping us, join us at the #mediaportal IRC channel at EFNet and ask a developer to help...
there's one other chance you have...
have you ever had a look at this mediaportal skin: https://forum.team-mediaportal.com/viewtopic.php?t=12781
i think this guy would be happy about your artwork because he's using licenced stuff and can't publish his skin to the public - maybe you can chat him and he's doing the skin-coding work for you with you as designer in background...
basicly you only have one image in that list (the one that shows the selection)...
so it's not really possible to archive what you want without code-changes...