I had this in my ToDo for a long time and recently I found time to take a look.
I’m just scratching the surface of the JATS’ universe (very chaotic one, by the way) and me neither want to reinvent the wheel, so the obvious first step was looking inside the PKP ecosystem…
I tested Open Typesetting Stack and looks really promising (congrats to @axfelix for the huge work) but some basic features (ie: table and image conversion to JATS) are still buggy… Any case, this is not the question (yet ), the point is that I finally arrive to the substance project (PKP is member of the consortium) and I found the code is under a free but non formal license:
I spoke to them about this some months ago – they asked us if we needed them to commit to GPL and we said no, that wasn’t a fair expectation, and they would be fine going with Apache / MIT / BSD. If you’d be satisfied with Apache as an alternative to GPL, mindful of the tradeoffs, then I don’t think you’ll find MIT to be too much different.