This is a nice little level editor I made. At the moment, it can only move and rotate objects and change some of their settings (string editing is broken). Adding/deleting objects may be added in the future. It can only save edited levels as byml files, you'll have to repack and compress them yourself.

Controls are:
Left mouse button + drag to move camera
Right mouse button + drag to rotate camera
Scroll wheel to zoom

Alternatively use these keys (holding shift makes them faster):
I,K: Rotate X
O,L: Rotate Y
P,;: Rotate Z
A,S,D,W: Move
Q,E: Zoom in/out

Here's a (outdated) preview of the editor: https://youtu.be/i5I19UA2FBQ
I'm now able to test custom stuff on my Wii U, so I made a video of a little edit I made to High Bounce Highlands. Watch it here: https://youtu.be/UzIFnSWh9yA

Requires Python 2.X (tested with 2.7), PyQt4 and PyOpenGL to run.