Handcoding all the way! I don't know what really is Skinux, but they seems to be a bunch of developpers. Nothing on their sites mention Skinfactory or anything on first look. Doing a WMP skin should not be intense to that level and it pretty straight forward once you get a grip of it.
Skinux developed a engine/code that skinsfactory uses regularly. [link]
as far as developing media player skins, my only concern is the placement/positioning of the image files and overlaying them over each other.
Do you know that you can exam a .wmz file by re-naming the file extension to .zip - then extract the zip to a folder? This allows you to see the graphic files and the .js as well as the xml. Well, thats what I do to get a closer look at the core of any skin. you should try that.
Do you have any xml programming experience?
anyway, stay in touch my email is jorenfx at gmail.com