Is there a way to do a costal cartilage graft without my breasts being visible, or touched, during the procedure?

Yes! Often times in revision or secondary rhinoplasty cases, you need support from costal cartilage. Traditional techniques involved using your own cartilage, but that will include an incision under your breasts and the whole dissection and its associated post-op pain. Another great option is using Fresh Frozen Rib Cartilage Allograft. This is made by the company MTF and they bank cadaveric costal cartilage and process it to create sheets for surgeons to use. The benefits are NO scars on your breast because you don't need to remove your own cartilage. It also speeds the surgery up as well because the cartilage is ready to go from the box. I use MTF cartilages for all my revision rhinoplasty cases and have found them to be very reliable.

I can understand and respect your concern of your modesty.
However during the physical examination and planning for surgery, your doctor needs to examine those body parts that assure good placement of the incision, and good field exposure to assure the best plan and results. That will include, in your case exposing the infra-mammary line and marking it which will include moving the breast to properly visualize the position of the incision.
In the operating room the field of surgery will extend beyond the planned incision to assure sterility of the field of surgery, and adequate exposure for performing the surgery in a safe and optimal situation.
In the operating room your modesty and body is treated with ultimate respect for your body and modesty. Your Board Certified Plastic Surgeon will treat you with ultimate respect for you , your body, and your modesty. but need to assure your safety, and best surgical plan. That is true also with the operating room staff.