Hello,
I never tried it, but in the General settings of the server you can enable a "Footer" and allow domains to override the default footer.
That's maybe what you should try.
You should maybe enable this like in this screenshot (general settings).
and leave the general footer to "None" and then you should be able to set a footer on a domain level, probably by impersonating the domain administrator of the domain.
Again, I've not tried it personnally, but looks like it might help in your case.
Kind regards
Sébastien Riccio
System & Network Admin
https://swisscenter.com