i too bought the magnificent mountains, the reason i bought them is to "dissect the scene" to see how it was done! and i would have paid that price for the textures alone (since bryce snow textures are for the want of a better phrase .....below par.
going back to your point, personally i agree with all of the above in saying that if you alter the scene in any way or add something to it, then it becomes a new scene. if on the other hand you fired up bryce imported the scene, rendered it and fired that image on to your gallery then that would raise ethical questions (not saying for a minute that you would by the way!) in fact that gives me an idea, i'm going to post my new image "mystical mountains" which is similar to majestic mountains except that the camera has been moved millimetres to the left (mmmwwwhhahhahahah)