For the dev part, use an IDE with Groovy support, and load in the classpath the Jenkins core lib and possible plugins you wanna twiddle on.
Maybe look also at the scriptler plugin.
In general, when you don't receive answers, it may help to rephrase or rethink the subject.
From outside, what you wanna do could look like what actually plugins are designed for: offering in an accessible way standard feature for all jobs and so on. So what you want is actually possibly developing a dedicated plugin for your additional features. Rewriting (or not, btw) your groovy script in Java would be very straightforward since this is actually the same thing you're calling.