No studies on mammary glands vs brain size in females. I would not expect much correlation in bats, since bats probably do not carry around extra tissue given the energetic constraints they face. The severe constraints faced by bats make them a favorite group for those scientists interested in parental investment in young and how it relates to mating behavior and ecological factors.
I would be careful in generalizing these results to primates. :-) A lot has been made in the popular press about these sorts of correlation studies. But as you and I know correlation may point to hypotheses, correlation is not causation.
Bats are among the most diverse groups of mammals in terms of biology and reproductive strategies. Curiously there is even a species of bat in which the males have been reported to produce milk, perhaps only 10% of the amount of milk produced by females. For more on bats see:
As for the lactating male bats, see this article on male lactation and references cited therein. It is not known if the male's milk is used to feed the young, or even if this male lactation phenomenon is adaptive at all but rather some sort of anomaly.