The Roman garb I made last summer is a breeze for breastfeeding, since the sides are open far enough under the arm to just pull the opening over the bust and nurse that way. If you wear a wrap/veil you can throw that over yourself and the baby if your baby is distractable and/or if you worry you're showing too much (though what could be more Roman than feeding a baby! They REVERED matronhood.).
I recently modified another chemise to use under another dress (see below) and this time took my inspiration from period images of nursing mothers like this one from the excellent What Nursing Mothers Wore page. The image is Virgin and Child by Giovanni Antonio Boltraffio (1493-1499)
My next project was one I'd hoped to get done before King's Road in July, a hangerok out of some red wool I have with a sort of shadow check throughout. I only have about two yards, so a hangerok's about my only option. I know there's a lot of debate and very little evidence about how these looked, but I found an image on Pinterest (everyone's favorite legit research site) of a hangerok with two overlapping fronts. Bingo. This will be perfect for breastfeeding over the white chemise.. Here's the image, from this pin from a blog with a bunch of broken image links.