As Jelle suggested, a custom theme should help you out. Alternatively, you could implement the script with a custom widget?
Alternatively, if that doesn't work for any reason: I don't know exactly how your seal system works and under which circumstances you're allowed to use it. But you could theoretically create a second html page (let's call it seal.html) without all of the Mendix jazz (mostly the script tags). Once you've let Norton verify that you host the seal you could download the image and include it in your Mendix theme.
We are using a custom theme and we added the code in footer! We even tried to use a HTML snippet.