R’ Elchanan Wasserman quotes a the Ran at the end of Kiddushin as explaining that women can say a bracha if they voluntarily perform mitzvos aseh she’hazman gerama because these mitzvos are obligatory on men and women receive reward for their performance. Why, asks R’ Elchanan, should it make any difference whether men are obligated in these mitzvos to the discussion of whether women can say a bracha? Furthermore, getting back to the Ramban’s chiddush (see post here) that the Avos did not observe the Torah in chutz la’aretz even as ainam metzuvim, R’ Elchanan asks why women should get schar for performining mitzvos she’hazman gerama in chu”l as ainam metzuvim? (R’ Elchanan obviously took the Ramban’s chiddush to explain Chazal’s derush very seriously!)
R’ Elchanan explains that receiving schar is not sufficient reason to recite a bracha – how does the language of “v’tzivanu” fit? The action involved must categorically be defined as a mitzvah. The Ran mentions the fact that men are obligated to indicate (a siman) that we are dealing with actual mitzvos. The only difference with respect to zman gerama between men and women is that the former would receive punishment for violating these mitzvos, the latter do not.
Based on this approach there is no question from the Ramban. The Ramban’s distinction applied only pre-matan Torah when the only issue involved was the degree of schar obtained for fulfilling ratzon Hashem – there was no concept of mitzvah which existed yet.
(R’ Elchanan’s chiddush suggests that there is a chovas hagavra on women to perform mitzvos which are zman gerama, which I found very interesting [please see it inside – I may be reading too much into it]. I would have suggested a Brisker-like approach: the command of Hashem is categorically defined as a mitzvah viz a viz men and women, i.e. the cheftza of the dvar Hashem is a tzivuy; however, the chovas hagavra is different. The problem with my idea is how can one say “v’tzivanu” when there is no chovas hagavra?)