I'm planning to create an index automatically and then fine tune the entries manually. I remember having done it in the past but now I cannot figure out how.
The way to somewhat automate index creation is with a script. Marc Autret seems to have recently updated his IndexMatic script. It's a free download from his website (which is in French, but the page I've linked has English instructions). IndexMatic allows you to create a starting index from a list of words, pretty much by brute force (the original script wasn't called "Index Brutal" for nothing!).
Maybe I will not address your question to 100%, but I found more general solution for making indexes. My clients often want me to create index from MS Word document with the list of words to be included in index. Of course Word has it's own tool for it but you have to put index words manually word by word. My colleagues also want me to create the index in their InDesign files, again based on the list of words. Here the above mentioned script IndexMatic is fine solution.
My general solution is to create index from pdf by means of free soft called Index generator. I tried it and it works, even with the words in my language, full of special signs (Slovakian). There are a lot of options there, you can also manually define which word shall or not to be included in the index. I even succeded to import the list of index word from one pdf document and use them in the other indexed document (pdf). You can also select the output in various formats....
If you are interested, I can explain it more detailed in some other comment or I will update this answer.