For these I used the 'fuzzy' transparency for the first material and then mixed that with an invisible mat with the distribution driven by a function. You can then use altitude/slope for the added layer of transparency.  I'm going back into Vue today for another round of experimentation and see what else I can come up with.  I've been playing with the metaclouds as well..stretching them way out for contrails etc....it works somewhat but you can't change the color schemes too much.