Is there a hook in the API that allows me to raise a CDI event when an index has been written to?

I would like to avoid any chance of a race condition and as far as I can tell I can determine when an index is about to be written to but not that it has definitively been written to and that any subsequent read is guaranteed to get the data.

Interesting idea.
Are you needing this because you need to index asynchronously?

By default it's synchronous, so as soon as your transaction is committed, queries will find the new elements.