OK, I've now played around with it and it works very nicely and will be easy to integrate into my software. You've already put a lot of work into this so I hate to point out any of my peculiar preferences, but just in case you feel like putting even more time into it here they are:
1. Add the 15va/vb marking as well.
2. I'm not sure what standard practice is but you'll see in the score that my software produces it does not extend the ottava marking over rests. My music tends to have a lot of rests (a consequence of how it automatically changes staves) so it has a big effect on the overall look.
3. Change the text. In my score I just use "8" and "15" which given how crowded the score gets at times makes a big difference.
My version is the Piano_Standard.pdf whereas yours is auto-ottavate.pdf
Thanks again for the work you've put into this!