This'll definitely need some special support for something like this. Since this is a common problem, saving files alongside the config.xml, please take a little extra time to make it a re-usable feature.

I tried to make it reusable for others to extend the plugin. See pull request.

Hi,

I was migrating to DSL plugin to manage all our CI and release jobs (~400) when I discovered that promotions are not yet supported, which is a blocker to implement our build pipeline.
In my case, promotions are configured in template jobs. Jobs created using these templates get any empty 'promotion' folder.
That would be great to support this.

Here is the link to Thomas' pull request (additional discussion in it) that may help to track progress on this subject : https://github.com/jenkinsci/job-dsl-plugin/pull/157

For community visibility, I recently rebased Thomas' pull request with the lastest updates to the plugin and added a small fix. This PR is now able to merge Thomas' work into master: https://github.com/jenkinsci/job-dsl-plugin/pull/291

I don't know if this could help but Promoted build plugin v2.18 introduces a new REST API to manage promotions (fixes https://issues.jenkins-ci.org/browse/JENKINS-8963).
Maybe DSL plugin could use it?

Seems like the promoted build plugin's choice to use external config files breaks every other plugin that tries to some generic task with config files, including backup, git snapshots, etc.
Would it be less trouble to migrate the promoted builds data into the main config.xml.

Yes, of course it would be simpler just to fill in all promoted build plugin config data in standard config.xml
Maybe there should be a task for promoted build plugin developers to change the way config data is persisted.