I have BF at weddings in two styles of dress.
One was strapless, zip up at the back - worked well, I tucked a cloth in the top of the dress (it was silk satin, and I got NO milk on it!), undid the zip a bit and popped my boob over the top to feed. Just don't get up and walk around while feeding her with your scary bra exposed at the back!
The other was a wrap style empire line top - not so good, I got milk everywhere on the top, and didn't feel like she had good access.
Hey - I'm the first to suggest over the top access!