Youd probably be better off dry aging the whole subprimal instead of just the culotte/picanha/namp184d. They're fairly thin and you wouldnt have much left after cutting off the exterior waste, and the best part of the cut (fat cap) probably wouldn't survive either way.
I've never seen it at costco with the cap still on, though I'm pretty sure I've seen "top sirloin roast - cap off" in the bin. Its vacuum packed so I doubt they butcher them on site which means they probably couldnt leave the cap on by special request.