The easiest way to add an index of songs is to duplicate the contents of the \s Song Titles using a Changes.txt rule like this:
'(\\s )([^\\]+)( ?\r?\n)' > '\\toc1 \2\3\1\2\3'
This says, look for a \s section heading and call that match-group 1.
Then grab the content/text of that heading, calling that match-group 2.
Grab the end of line (which needs at least an \n but might have a space and/or \r before the final \n) and call that match-group 3.
Then, on the replace-with side, put in the new marker \toc1 followed by the contents of 2 and the end of line piece. Then put everything else back as you found it (1,2,3).
These \toc1 fields should now get picked up and used in the Table of Contents.
Note that you can also generate a sorted (table of contents) from these same \toc1 markers for an index at the back of the song book. Just use \ztoc|sorta\* instead of \ztoc|main\*