The Velveteen Rabbi has a sweet post about fashioning a tallit for use during breastfeeding -- with details about how she devised clip-on tzitzit.
She also links to the Conservative Teshuva by Bradley Artson Shavit that concludes,
Based strictly on our read of halakhah, it is permissible to breastfeed in synagogue; based on our Jewish values, it is a positive value to make nursing mothers welcome in our services.If you are considering breastfeeding in synagogue, to be prudent, we suggest you consult your local rabbi, with the emphasis on local. The answer you get in Teaneck may be different from the one you get in Park Slope.
Many synagogue arks are emblazoned with the words Da lifnei Mi atah omed (Know before Whom you stand.) In Torah study and in prayer, we are in the presence of the One whose salvation is intimated through human nursing: “That you may suck, and be satisfied with the breasts of her consolations; that you may drink deeply, and be delighted with the abundance of her glory” (Isaiah 66:11). Jewish institutions, in particular, have an obligation to welcome, facilitate, and support nursing mothers and their babies.