I'd be quite keen on finding out more about this too - as the default rpm misses some of our internal standards and needs some tweaking.
I'd like to understand a bit more about your use cases. Part of the reason is that we are moving away from plugins as the mechanism to perform extensions. Instead of a custom RPM, I'd encourage you to use sandbox.jiveon.com (you can get an account here) and build an app. This will ensure there is a clean, upgrade safe way to get to the cloud.
If you can't do what you need to using our API and app framework, then this is a good indication of where we need to improve.
I'm not sure it's a matter of anything to do with plugins.
Our unix infrastructure is all backed with AD authentication - so the fact that the RPM creates a local user to run the services as is a violation of our standards for a start.
There are probably others too, but I've not delved deeply into everything the RPM does yet.
I know I am very late on this one but guess I can help you(or route you) on this one.
I think we can have other other custom ways of deploying the custom changes to your instance. Let me know if you are still looking out for this solution ?