Guava is a work of art created by the team that maintains it; what's the big deal if you can't add your own code to it? Merely a lost opportunity to advance your own vanity?

Respect their boundaries and let your experience inform your feedback to them. If you read what Kevin is saying, it is clear they are interested in hearing if, how, and why their library is helping or hurting your own project. They will probably listen if your feedback provides the answers they seek.