I doubt you intend to do this but I would like to be able to use your plugin on my server.

I have considered forking it and releasing it for Sponge but I can't find a license which means that this code is implied to be All Rights Reserved?

I don't have the time to release a sponge version at the moment. But if you want to fork it and add sponge support, you can release that with a link to the original project, or create a Pull Request and maybe it will be added to the original Brewery.