Sotah 24a - Mitzvah to drink

Often the mishna sneaks a chalutzah in the context of gerusha, without differentiation, almost hiding the fact that chalutzah is only passul to a kohen m'drabonon. Here too, a chalutzah married to a kohen is listed as a woman who cannot drink. The Tosafos Yom Tov explains that although there is a d'oraysa chiyuv to give her to drink, the Rabbonon were "oker davar min hatora" since it is a passive uprooting and they have the power to do that as the gemara explains in the 10th perek of yevamos. The Rashash voices a strong opinion arguing on the Tosafos Yom Tov, and explains that it is not necessary to use the concept of oker davar min hatorah here since the Torah does not obligate the husband or the beis din to give her to drink. The rashash writes:

דאין זה מצוה חיובית אלא דהתורה חדשה לנו תקנה להיותה מותרת לבעלה ע"י שתייה אם תנקה, לכן אם אינן חפצין בתקנה שומעין לה

Since there isn't any obligation to give her to drink, it is not considered uprooting a d'oraysa when the chachamim say that a chalutzah doesn't drink.

Perhaps the approach of the Rashash can be used to answer another question. Many meforshim (Maharatz chiyus, malei haroim, mitzpeh eisan) ask how the mishna can list a pregnant or nursing woman who violated the rabbinic institution not to marry until the child is 2 as one of the woman who doesn't drink. This seems to assume that the Torah recognizes issurei d'rabonon as people who are not allowed to be married to their husbands and therefore do not drink. This should be dependent on a machlokes R' Yehuda and R' Meir in Succah 23a, whether a succah on an animal is kosher since m'drabonon it cannot be used on yom tov, it is a succah that is unfit for all 7 days. R' Meir holds that since it is only unfit for use m'drabonon, but m'doraysa it is fit for 7 days, therefore it qualifies as a kosher succah. Here too R' Meir should hold that since m'dorasya this pregnant or nursing woman can be married to her husband, she should be able to drink.
Based on the Rashash, the question doesn't even begin. There is no mitzvah obligation to give her to drink, rather it is a right that the husband has. Including the pregnant and nursing woman in those who don't drink doesn't imply that the torah recognizes the prohibition, because there is not obligation to drink. Rather, under the circumstances where the rabbonon don't permit the marriage we consider her m'drabonon unfit to drink and therefore revoke this privilege and take it away from the husband.