I don't know of such a plugin, but I'm curious how you think it would be possible to build an index without opening the documents, either all at once, or one at a time.
No, I understand that atleast 1 of the documents needs to be open, however I have 18 documents in my book ( it’s a 672 page catalog actually, each document is a separate section of the catalog) and the files are so large that if I have them all open at once (even with the fancy dancy computer that I have) it causes my programs to either freeze or crash. If I could just have the book open and then index the documents 1 at a time that would be lovely.
From my explorations in Indesign it did not appear that you could do that. f I am wrong please let me know.
If it's completely impossible to create a full index of your catalog without crashing, here's a rough workaround.
Index each section separately, and copy the eighteen resultant indexes on under the next into Excel. Sort the list, copy it back to your catalog and apply formatting.
oof that sounds like the extra work that i was trying to avoid. but that so far has been the best solution i have come across.