bpm,
Are you saying that setting Chapter Properties >>>> General (option to 'maintain aspect ratio' and checkbox to enable resizing)... does not meet your needs?
My experience is that pages will scale with very good quality and maintain aspect. If the client's display uses a different aspect ratio, they will simply have some 'background' colour surrounding the Opus page. Publication Properties also provides some control and customization.
Other than that, you could script something to detect client's display (there is an Opus System variable for that), and then branch off into a different set of chapters tailored to each possible screen resolution. Not very practical. Also complicates maintaining or updating pub changes (edit time).
Here is an example of using different chapter page size... for purpose of Printing:
viewtopic.php?t=442&highlight=print+long+chapter