A SumatraPDF forum expert wrote:
In theory you can write your own with say VB or AutoHotKey since all you need to provide to SumatraPDF is the -plugin argument i.e. numeric handle of the window you generate first. so should be easy for DOpus to handle that call.
This sounds like a great option. Thanks for this idea. So if I call "Sumatra.exe -plugin ", it will load into the viewer pane? I would check with DOpus how to automate this call whenever the viewer pane is activated and I've selected a PDF file.