There's quite often talk about plugins, hooks and listeners and other such sorcery and that point it does seem to be a lost cause if like me you can't program an add-on.
However just to put any other potential XenForo users minds at rest, I want to report that I have done this today without any problems. What I did was to make use of the excellent included advert templates to include the internet's best cheat: An iframe. This iframe pointed to the PHP file that generates a random banner every time it is called.
Job done, save for some custom PHP to float the banner to the right and to add some padding.
PS. I also did a "sponsors online" info block easy enough in the side bar. Just copied and pasted the staff on-line template code and changed the part where it checks for mod/admin to instead check for membership of a specific usergroup as per Brogan's FAQ. Job done.