At the best of my knowledge currently is not possible (correct me if I'm wrong) to set the environment variable BACKPORTS to yes for the debian-glue-non-voting job in the integration/config repository.
My personal use case if for cumin repository, that has a debian branch and is set up to run the debian-glue-non-voting job on every CR on that branch, but it also requires to have BACKPORTS=yes set in the environment for the build to succeed.