There is actually a discussion on Google Groups if it makes sense to add support for the Job DSL plugin directly into the Promoted Builds Plugin:
I created already a Job DSL Extension in a separate Repository (https://github.com/codecentric/job-dsl-promotions-plugin). The question is now if I should invest the time to create a PR for the Promoted Builds Plugin. The main point is here, that the extension is mainly written in Groovy. Is it ok for you to see Groovy in your code base?