If you get the new FilterCmd add-in v1.1 which I just updated, you can run this from outside of Opus, and the progress dialog will behave similar to what you'd get from inside of Opus:
"C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /acmd FilterCmd FILE "file1" "file2" "file3" CMD Image HERE width=1920 height=1920 NOENLARGE PRESERVEASPECTRATIO QUALITY=90 REPLACE=always
Split up so it's easier to see (but everything should be on a single line when you run it for real):
"C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /acmd
FilterCmd FILE "file1" "file2" "file3"
CMD Image HERE width=1920 height=1920 NOENLARGE
PRESERVEASPECTRATIO QUALITY=90 REPLACE=always
The file paths must be full paths, not relative paths or filenames on their own. If they contain spaces, the need "quotes" around them, otherwise quotes are optional.