It says on the second line of my original post that you do have to be root to install scripts in postwwwacct, however if you can get your host to install the code and give them the package names you want to force ads on they should be able to setup without a problem ...
Its possible to get php to insert headers and footers on pages automatically, but actually it breaks even more sites than mod_layout does, and there are other ramifications, like the ability to serve xml, and most importantly, if you're not root the only place you have to edit the php runtime is each webroot .htaccess, which can be changed by the user, and will be, quite easily ....
I think mod_layout is the only real solution to forcing content into sites.... maybe one day I'll come up with an idea and write something, but for the moment work with what you have ....