I was just looking into this recently. You can do it in C++:
/** Override the skybox used in the compositor (e.g. for during level loads when the app can't feed scene images fast enough)
* Order is Front, Back, Left, Right, Top, Bottom. If only a single texture is passed, it is assumed in lat-long format.
If two are passed, it is assumed a lat-long stereo pair. /
virtual EVRCompositorError SetSkyboxOverride( VR_ARRAY_COUNT( unTextureCount ) const Texture_t *pTextures, uint32_t unTextureCount ) = 0;
/* Resets compositor skybox back to defaults. /
virtual void ClearSkyboxOverride() = 0;