Custom div as a background for all slides (Feature)

[JerryI]

RevealJS allows to put background images, iframes for all slides.

However, it seems there is no way on how to put a custom DOM element. It would be cool to have sort of a master slide for instance, which will underlay all other slides.

<!-- underlay slide -->

Footer
Logos
Pics in the background

<!-- normal slides -->
# Title 1

---

# Title 2

or something similar

[JerryI]

Is there an API for custom slides?.. I could find it in the official docs, or does it break the architecture of revealjs?