It depends on the type of feature you would like to implement.
But if it's low level C code, and if you think it might be useful as a global feature, you could send a PR and maybe we can merge it into the core code.
Otherwise if it's a niche feature that shouldn't be included by default, the fastest way to get started is to just fork the project and add your
component depending on what you're trying to do. The Java code very much resembles the iOS counterpart in how actions and components are written, so you could do the same. I haven't written an instruction for the Android side (I should, but haven't had time)
As for JEM, it works as advertised on the github repo, and it should be pretty straightforward to get it to work but I can't say with confidence that it's "production ready", but you could try it.
If you can share what you're trying to do, maybe we can come up with the best approach.