The utility would be kind of limited, I think. Unless you're asking peppy to go ahead and code an entire graphical paint program that rivals GIMP or Photoshop/Illustrator or Flash or what-have-you, odds are it's not going to get a lot of use.
That got me thinking, though - rather than an In-Game editor, wouldn't it be handy if there was a skin previewer? Basically a standalone client that allows you to test each of the images in a pseudo-game environment, such as:
Test hitcircles:
(optional parameters)
-Force 300
-Force 100
-Force 50
Test Spinner
Test Slider
(optional parameters)
-Repeats: n
View Ranking Screen
View Pause Menu
View Song Selection Menu
etc~