@Ryn: Do you know that Sigil has a built-in index tool? To use it, all you have to do is generate a plain text file with index entries, e.g. index.txt, and select the following:
- Tool > Index > Index Editor...
- Right-click > Open > index.txt > Save
- Tool > Index > Create Index
Obviously the index entries won't have page numbers, but you might be able to add them later with a custom Python script or a plugin.