Is it permissible for a woman to stipulate in the marriage contract that she not be prevented from supererogatory acts of worship, such as fasting on Mondays and Thursdays?

The basic principle is that a woman has the right to stipulate in her marriage contract whatever she deems beneficial for her, as long as it does not contravene the ruling of Allah, or permit what is forbidden, or forbid what is permissible.

Stipulating voluntary fasting, such as on Mondays and Thursdays, is permissible. This does not contradict the purpose of the contract; rather, it makes the husband's permission for fasting obligatory.