There is no scholarly consensus behind this. No doubt that a woman can leave a marriage if she is unhappy and she and her husband get to the point where their differences are reconcilable. However, polygamy is already permitted in Islam as long as Muslim men can fulfill the conditions attached. And the same way the woman can demand the clause to be added to the nikaah, the Muslim man can also deny it on the grounds that it is already permitted in Islam.
I follow the Hanafi school of thought and also agree with Malaki as well in many areas. I have never heard neither jurist endorsing what you and the lady claimed.
How are YOU saying there isn't a scholarly concensus when many scholars have spoken about this and even the sahabas knew about this? Even imam Malik commented on it and said it was common for the woman of Madina to do this!
This condition does not make something permissible forbidden; rather it restricts the man’s power and gives the wife the right to annul the marriage. Such conditions were made at the time of the Sahaabah (may Allah be pleased with them).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) was asked about a man who married a woman, and she stipulated that he should not take another wife or make her move from her home, and that she could live with her mother, and he went ahead (with the marriage) on that basis. Is he obliged to adhere to that, and if he goes against these conditions, does the wife have the right to annul the marriage or not?
That is because of the report narrated by [al-Bukhaari and Muslim] in al-Saheehayn, that the Prophet (blessings and peace of Allah be upon him) said: “The conditions that are most deserving to be fulfilled are those by means of which intimacy becomes permissible for you.” And ‘Umar ibn al-Khattaab said: “Rights are connected to conditions.” So the Prophet (blessings and peace of Allah be upon him) made that by which of means intimacy becomes permissible more deserving of being fulfilled than anything else.