Developing metarunners in a branch with vcs settings

We use build configurations based on metarunners that handle our CI builds and CD deployments. I'd like to be able to work on the metarunners to extend their functionality, ideally by branching the VCS repo for settings, and then executing custom builds using the branch name. That _almost_ works, but I'm told that metarunners are excepted--presumably you always get the metarunners from master, not from the feature branch.

Is there a workaround for this, or some best practice? Or can I work on metarunners in branches and expect custom builds to use the branched code everywhere, including in the metarunners?

Thanks