The term Brownie points came to mean accumulating points for doing good things and is a positive expression.

Not necessarily positive, as I understand it. It can also mean measuring "merit" by an compusive, computational and mechanical toting of minutia.

Anyone else familiar with the derogatory usage? Is this an area where perhaps British and US usages differ?

Dating may be a clue. The US Girl scouts were founding in 1912 and, as I understand, the Brownies somewhat later. Does the term brownie points predate this?