I've went through the exact same thing at Chateau Wyuna (gorgeous place!). Except that my DS was only 4 weeks old when I was bridesmaid. Luckily he didn't need to be fed during the ceremony but I had EBM in case he did. I asked my mum to come along to the ceremony to look after DS and DD.

I got him used to having EBM from a bottle from birth and mum gave him that while we were off having photos. Straight after photos I bf'd him and did so as well during the reception. The staff were lovely and I used the bridal suite to feed as I had to literally pull half my dress down to feed.

DS is 5 months old now and he would have about 200ml of EBM.

Oh and I was not able to find a strapless bf bra
Because it was only 4 weeks after giving birth I ordered my dress in 2 sizes bigger than I would normally wear and got it taken in the week of the wedding.

Hope that helps!