If you search Google for
"Mrs. * ZT-L", you'll find many instances of this honorific used for couples, and a few for women. Here are some examples of it used for women by various Jewish news or public relations outlets:
BaltimoreJewishLife.com regrets to inform the community of the petirah of Mrs. Chaya Bobrowsky, zt’l, grandmother of Reb Yoni Adler.
- "BDE: Mrs. Chaya Bobrowsky, zt’l, Wife of TA's Venerable Rebbi, Rabbi Yaakov Bobrowsky", BaltimoreJewishLife.com, March 22, 2016
Generation Sensation was created in honor of Rebbetzin Leah Raichik ZT”L, the first shlucha of the Rebbe in California, and Mrs. Miriam Weiss ZT”L, a pioneer of the Los Angeles frum community, who both have grandchildren and great-grandchildren enrolled in Bais Chaya Mushka.
- "Event Honors Moms, Bubbies", COLlive Community News Service, May 14, 2015
The proceeds from the dinner, as well as the commemorative journal saluting the school and the honorees, will go directly toward the Sarah Rivkah Lander Chesed Fund, established by the Lander family. Mrs. Sarah Rivkah Lander, zt”l, was a vital partner in the historic achievements of her husband, Touro Founding President Dr. Bernard Lander, zt”l.
- "Lander College for Women Celebrates 40th Anniversary", Lander College for Women News and Events, July 2, 2014
I think it's fair to conclude that while this honorific is probably used a great deal less frequently for women than it is for men, its use for women is not unheard of.