Here's another idea! They have since replied with a lot more info in the Sumatra forum leading me down some other paths.
They mentioned a couple of others that utilize Sumatra as a plugin.
1.) Far Manager (text based file manager)
2.) Total Commander (Formerly Windows Commander, $46, file manager)
3.) Universal Viewer ($26, file viewer of many many types, PDF viewing based on Sumatra)
As I was looking into #3 above I discovered this link. (Sumatra PDF based plugin 0.8.1 - Total Commander) It’s the plug-in for PDF viewing utilized by Universal Viewer. Is this something that could be easily integrated into Directory Opus’ preview pane? Another idea I had is could I buy Universal Viewer, and use it for all preview tasks within Directory Opus?