Leo,
Per your recommendation I posed this question in the Sumatra forum and they have responded.
They suggest that although it is possible to implement within Sumatra, it is also possible to implement using the Sumatra engine within DOpus. They provided an example of another file manager (FileVoyager) that utilizes the Sumatra engine to load a preview and it is still then able to use Zoom within the preview pane.
Would this be a lot of work to implement? It seems similar to the way that image viewer pane zooming is handled?
Byron