I've had to breastfeed in a strapless dress twice- once for each of my sisters' weddings. These were formal dresses that I couldn't even unzip without assistance. With the first wedding, I nursed my baby right before I put the dress on and then again right after pictures, before the ceremony. I was able to find a private place which was necessary because I could not feed discreetly (or even almost discreetly) in the dress. I changed fairly quickly after the ceremony because he needed to nurse again and the inside of the dress was SOAKED in breastmilk (my son was just a couple weeks old). My sister completely understood why I changed and was perfectly fine with it.
The second wedding was outdoors with no chance of privacy unless I went off in the woods and found a log to sit on, but that might've ruined the dress... I had a cardigan and my husband helped me unzip so I could nurse and used his suit jacket to help me cover up a bit. The issue was having to pull a boob out the top of an unzipped dress. I can usually just lift my shirt and since baby covers my breast and tummy, nothing shows. It really was not so discrete, but the baby got fed and nobody said anything.
My advice would be to find some privacy or see if you can change once the ceremony and formal pictures are done. I'd probably even use the bathroom to nurse in if it were quite clean (gasp!). Not that I think you should have to use the bathroom- I just would've preferred that to being partially clad in front of 100+ people. Not my favorite breastfeeding memory.