Payment processors are a valid concern, I agree.

The obstacle then becomes not putting any emphasis on such parts in ad campaigns, so that you can help to avoid issues that might arise with PayPal and others. As long as the ads aren't sexually themed I don't see PayPal really having much issue with it, or even really noticing it. There are plenty of sexually themed items available here at rosity that paypal doesn't seem to care about, because its done in a tactful enough way to make it seem more "innocent" than it really is.

I guess the safest approach, for the design of the base figure, would be to just include enough geometry in that area for others to build what they want, which is exactly what many have suggested. 

You could even go so far as to actually sculpt/model the correct anatomy, to be sure you have all the geometry needed, and then smooth it all out before rigging.

You could also use geo-swapping - also suggested several times - and make one hip with and one hip without. It's always been acceptable for male figures, there's no logical reason for it not to be acceptable for females, other than pure blatant gender discrimination. 

Personally I tend to run into problems when dealing with the creation end of geo swapping, and it can cause issues in other rigging/morphing areas, but it is one alternative. 

You could also simply offer two versions of the same model, identical in every other aspect, and use the "PG" version for all your ad campaigns. 
 

The thing is, plenty of sexually explicit models and add-ons already exist, and have large customer bases, and payment processors that don't mind, so it can be done. 

 

~Shane