Is it permissible for a woman on her menses to sit in the Mas’a?

Yes, it is permissible for the woman on her menses to sit in the Mas’a, because the Mas’a is not considered part of Al-Masjid Al-Haram. In that case, if the woman's menses started after the Tawaf and before the Sa’i, then she is permitted to do the Sa’i because the Sa’i is not the Tawaf.

Also, to be in the state of purity is not a condition. So, based upon this, we say that if the woman on her menses was to sit in the Mas’a waiting for her husband, then there is no harm in her doing this.