I came across in interesting shitta in teh Sefer HaMachria (by the Tosafos RID) Siman 78. Rabbi Akiva Eiger quotes this particular shitta in regards to whether a blind person is chayav in a lo saseh. The Machria writes a blind person is patur from all mitzvos including lo saseh. (Rabbi Akiva Eiger disagrees). However, that is not the biggest chiddush. Here is the chiddush.
First some background: Rabbeinu Tam (Kidushin 31 and other places) writes that a woman can make a beracha on a mitzva she zman grama. A rayah is from suma where one can infer from the gemara in Kiddushin that a suma can make a beracha even though he is patur from mitzvos. The Machria rejects the rayah for the following reason. (Tosafos in Eiruvin and Rosh Hashana rejects the rayah from suma for other reasons). If a woman would do a mitzva she is not chayav in b'toras chov then she violates Ba'al Tosif. Therefore, the rabbanan can never be m'chayeiv a woman in a mitzva d'oreisa because by doing so they cause her to violate Ba'al Tosif and they have no right to be okeir the lav of Ba'al Tosif. However, she can do the mitzva b'toras reshus. However, if she would make a beracha on the mitzva she is showing she wants to do it b'toras chov and m'meila she would be oveir on Ba'al Tosif.
A suma on the other hand is patur from all lavin. He has no issur of Ba'al Tosif. Therefore, the rabanan can be m'chayav him in mitzvos and he can make a beracha. Doing a mitzva which he doesn't have to do b'toras chov doesn't violate Ba'al Tosif since he is not chayav on lavin.
This idea that a woman is oveir Ba'al Tosif for doing a mitzva b'toras chov, and m'meila the rabanan can't be m'chyeiv her to do it, is a chiddush.
I just want to add that when I wrote "the rabbanan can never be m'chayeiv a woman in a mitzva d'oreisa because by do so they cause her to violate Ba'al Tosif " is my own hesber in the Machria. The Machria doesn't differentiate between the rabanan being m'chayeiv a woman in a d'oreisa or being m'chyeiv her in a din d'rabanan or takana/gezeirah d'rabanan like netilas yadayim. But to me he can't mean they have no right to be m'chyeiv her in a din d'rabanan. It makes no sense to say that. That is why I think he means they can't be m'chayeiv her in a d'oreisa.