Karsten,
Thank you for the message.
The MobileFrontend extension doesn't support footer customization, except through other extensions (in this case the ContactPage extension) using the "SkinMinervaOutputPageBeforeExec" hook (
history). The ContactPage extension would need to be modified to implement this hook in order to get it working alongside MobileFrontend for the mobile web presentation.
The Wikimedia Foundation is not actively developing the ContactPage extension or reviewing patches for it, excepting security bugs if they're identified. I'm sorry this isn't terribly helpful. One way such features are sometimes built is through a couple of volunteer engineers. Typically one person writes the patch (e.g., for ContactPage), and then another who has elevated permissions to sign off on the code reviews and approves the patch.
-Adam