I had this same problem with a halterneck dress I wore to a wedding in May.
I can't offer much help with the bra sorry. In the end I gave up looking and just expressed enough for the day, put it in milk storage bags, put those in a small cool bag with ice pack to keep it fresh and took sterilised bottles and a small water flask! It was a bit of a faff as I am so used to not needing all those extra things but it worked for me in the end.
When I think back it wouldn't have been feasible for me to BF Toby anyway. There was no private room I could have gone in to and there was no way I was un-doing the halterneck and taking my dress down to my waist to feed him!

I hope you find something. Have you looked on figleaves.com? I forgot to look on there when I was searching for one, maybe they might have something?