Leigh, I love that picture!! That makes it look so easy!
I found it easiest to nurse DD in the ring sling while wearing a nursing tank under a button down shirt. She only liked to be carried upright so the sort of nursing that Leigh and Wendy are demonstrating wouldn't have worked for us. I'd just sort of loosen up the fabric to position her lower down by my boob, unhook the tank, latch her on, and toss the tail over her. I imagine I could BF DD in the Ergo but I'm usually carrying her in a back carry. Not sure how that would work ...
I definitely agree that what you wear clothing-wise can make or break the success of nursing in a carrier! In my experience, you want something that can be pulled down/to the side, not up from your waist (since the carrier and baby will be pinning down a lot of the torso of your shirt). I lived in my nursing tanks, and/or nursing tank or bra + t-shirt/dress with a very stretchy neckline.