I'd wear a nice dress that was suitable for BFing, think 'BFing' clothes are generally horrible. I'd say you might end up exposing more than you'd really want with the shirt dress unless you plan to wear a vest underneath that can be pulled up/down as appropriate.
Watching with interest. I need a dress for a wedding. Love the shirt dress but pricey for viscose. i think i will keep a lookout for shirt dresses as they van be worn again and I think specific breast feeding dresses are pretty ugly.