You are the first person who is asking for the JSON serialisation :) Why do you need that, I'm curious?
At NimbusDS we only make use of the compact serialisation, to work with JWTs.
Feel free to have a go at that, and if you have any questions just let me know, I hope I would be able to help.
Vladimir
We now have support for JWS JSON, currently in preview. The following article and examples explains the API and how to use it with multiple signatures:
https://connect2id.com/products/nimbus-jose-jwt/examples/jws-json-multiple-signatures