I know I am being ultra-dense, but for the life of me can't find reference here or elsewhere (Help) as to how to set up the viewer pane to view epubs etc. with a simple standalone viewer like STDu. Does there have to be a plugin provided by the software authors? It's the ONLY one of my most regularly accessed file types I can't readily view without opening an external program.
TIA for any assistance! 
Things have to be specially written so they can be used inside of another program, so basic standalone viewers won't work.
Opus supports lots of different types of embeddable viewers, including preview handlers (the same used by Windows Explorer and Microsoft Office), and ActiveX controls (as used by Internet Explorer), as well as Opus's native plugin API. Many viewers support at least one of those, but not all do.
If you can view the files in Windows Explorer then they should work automatically in Opus.
If you can view them in Internet Explorer then they should work if you add their extensions to the ActiveX plugin (on either the Internet Explorer or Generic ActiveX lines).
Sometimes a bit of extra configuration is needed on the viewer side. (Some examples are here.)
If in doubt, ask the authors of the viewer which types of embedding (if any) they support.
Thanks, Leo. I'm pretty sure someone else would have figured out epub viewing in DOpus by now if there is a way, but I can't find anything in the forums to support that theory! I will contact STDu.