How to access native view, from the PCL code of a custom view, or at least communciate with it

I have a custom content page in my app, which handles all the navigation.

I want to be able to to talk to the renderer of a custom view, to invoke a tight interface of messages on it, i.e. ShowZoomed, PresentPage, and some others.

I would assume that this is fine, as, as long as my renderer adheres to this interface, I could communciate with it in this way; but I can't work out how to get a reference to the renderer from my custom content page. Something like:
((INagivationStuff)myRenderer).PresentPage(page);

I know I can do this with messages; but I'd rather just do it by interfaces if the facility is there, as that seems cleaner to me.