Given that everything is a feature in Graphiti, needing to implement it as a feature seems logical to me.
The API reference of features does not list a zoom feature, so I think it doesn't exist, and you should make it a custom feature.
At a more conceptual level, I wonder why you need zoom. The outline view gives you an overview of the editor space, and your normal view is used for actual editing. Imho the latter becomes less useful if you start zooming it. Text with a size like 5pt or 20pt doesn't work as nicely for editing as the usual 10pt. If you zoom but without changing the text size, your text-elements shrink or grow in your picture while zooming which seems to be a not so nice property either.
Wouldn't it be possible to split your editor contents into several different editor views, eg like the drilldown feature in the tutorial?
I'm trying a very similar approach, but directly in the GraphitiScrollingGraphicalViewer, so it works both for the DiagramEditor and the DiagramComposite. However, scrolling with the mouse wheel provokes bug 414437 (https://bugs.eclipse.org/bugs/show_bug.cgi?id=414437) really often, so I'll try and work on both of them at once.