Well a reflectionTexture does not behave like a regular texture as the texture coordinates are computed based on viewer position (to get a reflection :))
The way the reflection is built depend on the texture.coordinatesMode property:
OK, so I think I have to use a CubeTexture with SKYBOX_MODE.
Is there a way to create a CubeTexture with one or more DynamicTexture instead of the path to 6 jpg files?