May I (again?) remind you that at least in the rpg books it is said that men and women in the faith are mostly equal? At least in the ranks below the top of the church.
And if in fact some of the "most devoted" are women than this certainly give the women a stronger position than (f. e.) women did have in the catholic church...
Martin himself did not say any explicit about this theme as far as i know. And as long as this did not change in books which might follow the rpg books are the only sources which say something about this topic in a more broader sense.
Everything beside this is only guessing based on a more meager data base (given the fact that few men/women of the faith are mentioned with a little bit more details) - but m. e. not enough to make a valuable statement about the situation for all Westeros which goes against the saying of the rpg books.
Of course in the end it is the decision of every group how they deal with this matter.
Concerning the main topic of the thread I am not certain. For now I see no indication for female knights - allthough the more...common form of knighting (simply by annother knight) might give some opportunities...
But it is still the question if such an action would be accepted by other knights and the faith...
Allthough i must agree that this thing with Brienne is a little bit open for interpretations.
To be fair, while some of the "most devout" may be women, we really have no idea about the structure or demographics of said group. Certainly, they sit in on discussions (likely as mouthpieces for the Mother, Maid, and Crone), but even if you assume equal representation by presumed gender of the seven, they're still outnumbered by the men (the Stranger, being the genderless representation of death, is likely either male, or at best for women's sake, a silent sister who's sworn an oath to not speak).
So, for the "most devout" to be a position of actual strength (as opposed to a feel-good title to give an illusion of power)
- representation demographics has to be equal (or at least possibly equal if it varies back and forth)
- said representation has to have equal rights
- said rights must include some sort of voting rights
If any of those fail, then septas are at best, a token presence in the "most devout".