Is there a book, preferably freely available online, that lets you look up a topic alphabetically and get a list of references to passages in the Zohar discussing that topic?
There are several possible suggestions. The most obvious is the alphabetical index of subject printed in the back of the Vilna-Romm edition of the Zohar. It isn't exhaustive, but it's a good place to start. The link from HebrewBooks.org site:
Two other excellent sources are: Pardes Rimonim of Rabbi Moshe Cordevro, Sha'ar 23, Sha'ar Archei HaKinuim. Again, alphabetical and by subject. It gives brief definitions and then indicates sources. It isn't restricted to Zohar, but there are many, many pages references. It starts on page 233 of the following link: http://www.hebrewbooks.org/19622
A biography about Rabbi Corvero is at this link. https://en.m.wikipedia.org/wiki/Moshe_Cordovero
And Sefer Meori Ohr by Rabbi Meir Papirosh HaKohen. The link from HebrewBooks.org is: http://www.hebrewbooks.org/7128
Rabbi Meir was born in Prague in 1624 and with his family moved when he was around 13 to Eretz Yisroel.He learned Kabbalah from Rabbi Yisroel Ashkenazi and Rabbi Yaacov Tzemach and had direct access to the writings of Rabbi Chaim Vital as preserved by his son, Rabbi Shmuel Vital as well as what was collected by Rabbi Yaacov Tzemach and also the final version of Rabbi Chaim Vital's writings which were buried with him. His gift was at systematizing and ordering diverse writings of which Sefer Meori Ohr is only one example.}