I’ve stumbled upon the following page, which explains why Hippo does not support SQL2: https://www.onehippo.org/library/development/query-the-repository.html
Especially since AEM has supported SQL2 for years now, I was quite surprised by this decision. I can also image that at some point both XPATH and SQL will no longer be supported by the JackRabbit maintainers. I hope you’ll reconsider (or have already reconsidered) this.
Additionally, I have a question: does the fact that SQL2 is currently not supported simply mean, in practice, that it’s not supported by the CMS repository web interface, or is there more to it?