The RH scene at https://playground.babylonjs.com/#UU7RQ#1956 shows the _nz image of a CubeTexture in the negative Z direction. The _pz image is shown in the positive Z direction. To me this is then as expected. I thought I had the flipped situation earlier. I had used explicit files which is what I try next on the playground.
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Sorry, it looks like I was confused about which way the pointed end of the capsule is directed. Adding markers at the origin and at (0,0,-10) shows that it is actually pointing towards positive Z (from -10 towards 0):
https://playground.babylonjs.com/#UU7RQ#1960
https://playground.babylonjs.com/#UU7RQ#1961 (with PositionGizmo)
This then means that the CubeTexture _nz image is actually shown in the positive Z direction, in a RH scene. This behaviour would then be unexpected, and either benefit from documentation or from being considered a bug.
Apologies again for the confusion.
Adding @bghgary the king of handedness 
No, no, handedness of the king. 
As far as I know, cubemaps are always in left-handed. This can’t change because it’s the convention of cubemaps and has nothing to do with the engine. If the engine mucks with this, the textures won’t line up anymore. I found this that has some explanation. I guess documentation doesn’t hurt here.
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The Khronos wiki does say that a cubemap’s axes are always in a LH system. I think that just defines in which direction the u and v for each face goes, and perhaps not necessarily where the pos Z goes but I cannot grasp that clearly without more figures.
In any case, it is clear that currently using a cubemap without modification in a RH scene will result in showing the backwards face (nz) in the pos. Z direction which is the backwards direction in a RH scene. For me the forum entry here suffices as documentation.
For completeness, here is the playground with explicit files for each direction:
https://playground.babylonjs.com/#UU7RQ#1962
It shows the same result, eg. is consistent with the faces inferred from the filename.
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