This is a copy of the current Skybox that can be used for a z-axis-up coord system.
With an additional flag on the constructor we could have both with correct texture coordinates.
http://cvs.world-of-mystery.de/cgi-bin/cvsweb.cgi/wom/src/de/worldofmystery/client/MySkybox.java
Wouldn't we also need to rewrite it so that North was still north and so forth?
North looks to positive y, east to positive x, up is positive z. It looks right here.
Sorry, didn't see the if in the code. 
Still, if we want to expand SkyBox, we might as well go to the effort of allowing more than just y and z up axis. Perhaps by specifying an upvector rather than a single boolean?
I tried to give it the camera vectors and rotate according the up and left vectors but I failed to find the matrix functions to keep it that flexible.