I just wanted to mention that our software KNIME Server is also using Mason for quite some time now (don't ask why I didn't post it earlier...). An introductory article can be found at https://www.knime.org/blog/giving-the-knime-server-a-rest
. I don't spend much time on Mason there but the main reason why we have chosen Mason was simply that it allowed us to use the existing JSON format and simply add the metadata elements.
Cheers and keep up the great work,