Blanket discount a transaction except already discounted items?

We have a group of customers who get a 15% discount on purchases of non-sale merchandise.

What I'd like our cashiers to be able to do is apply the discount and have it do three things:

1) Discount regularly-priced items 15%

2) For sale items where the sale discount is less than 15%, apply the 15% discount to the item's regular price.

3) Ignore items already discounted more 15% or more.

Does that exist in RMS? If not, does a plug-in/extension exist?

Thanks!