I don't know how DAZ/Power get around it, since they explicitly advertise models of real people, but a good way of being able to make nude models of celebrities, without actually making nude models of celebrities, is by making a "parody" in a sense. Much like how video game developers like R* put parodies of real life celebrities in Grand Theft Auto V, I can imagine you could do the same without getting into any legal trouble. R* only does get targeted because they're basically top dogs in the industry, they're going to be targeted because the hack celebrities who do decide to sue are looking for a quick, fat paycheck, regardless if they have a case. It's just how it is in Corporate America, we're sleazy as hell.
So say you have a HUGE crush on a porn star and you want someone to make a model for you in Garry's Mod or SFM to pose with. Find someone who is good at making likenesses of real people and tell them you want a model called... say... Roxxette Devil. You want to base her likeness on Former Porn Star Roxy Deville. You can give the modeler you commissioned reference pictures of the actual porn star, and if he doesn't want to model the porn star directly, he can change some features about her such as hair style, eye color, maybe thinner eyebrows, a nose that sticks out less, more rosy cheeks, maybe freckles (or lack thereof) or maybe a slightly thinner or thicker body type.