The hardest part is obtaining the Base UI Skin data from the compressed game files. However, most of the Base parts general sizes are all that's really needed.
.TGA files have to be masked, which is part of the saving process...so the tutorial would be the most important part.
Edit: I think the best time to do something like this will at launch time, when Turbine nails down the full, complete list of ArtAssetID's that we can Skin, as well as possibly unleashing something like a "UI Pack" or something for Skinners to use as base images. Not that they will do the 2nd of the two thoughts mentioned, but they did say something about letting us have a complete "dictionary" of ID's we can skin. The pack can be created at a later time.