Is it permissible to pray Istikhara concerning marriage to a niqabi woman, and would the Istikhara be about the niqab itself?

Istikhara (guidance-seeking prayer) is not performed for obligatory acts. Wearing the niqab is obligatory for women, hence Istikhara for it is impermissible. As for performing Istikhara for marriage to a woman who wears the niqab, if it is for the fundamental act of marrying that particular woman, then it is permissible, because marriage is a permissible act. However, if it is solely because the woman wears the niqab, then it is not permissible, because the niqab is obligatory, and there is no Istikhara for it.