The FAI.me build service now supports packages from the backports repository. When selecting the stable distribution, you can also enable backports packages. The customized installation image will then uses the kernel from backports (currently 4.14) and you can add additional packages by appending /stretch-backports to the package name, e.g. notmuch/stretch-backports.
Currently, the FAIme service offers images build with Debian stable, stable with backports and Debian testing.
If you have any ideas for extensions or any feedback, send an email to FAI.me =at= fai-project.org