Get a basic rack and use some p-clamps to secure it to the stays. P-clamps have a rubber jacket, so unless the stays are carbon, I can't imagine you will damage the frame.
Or, get a rack that secures to the brake bosses. Old Man Mountain makes a few, I'm sure there are other vendors or ways to do that as well.
Or, use a trailer. Weight has a minimal effect on performance, especially in a touring context. This would likely end up the most convenient option, especially since you can use it to haul stuff around after the tour.
Get the widest tires you can fit on the bike. I'd also consider using a stem to raise the bars for the tour.
15 lbs is probably not going to be enough, even for a CC tour.