Sacred thread (Janeu) for women?
In ancient times, most women would have no education in the Vedas, and they would just become housewives. But in contrast to these women, known as Sadyovadinis.
There was a very small number of women, known as Brahmavadini, who would abandon the material lifestyle that was the norm for women and instead went to an Ashram and learn the Vedas just as men did.
For these women, the Harita Smriti prescribes the Upanayanam or sacred thread ceremony:
According to Harita Dharma-Sutra the sacred-thread ceremony has been declared necessary for brahma-vadini or girl-students.
They should beg also from their homes while studying Vedas and performing sacrifices, etc. (Harita Dharma-Sutra 30-21.22) Ladies having no sacred thread were called 'Sadyo-vadhu' and thus they remained unpolished.
A lot of wives of sages, like Agastya's wife Lopamudra and Yajnavalkya's wife Maitreyi were Brahmavadini.
In fact Lopamudra and Maitreyi were actually two of the seers of verses in the Rig Veda.