Well, today I've had Directory Opus running all day, usually with multiple image viewer windows open. For the last few hours six image viewer windows have been open, as I've been referring to map images while writing a report.
With all these images open in Directory Opus' viewer, I've had no problems whatsoever with WIA hogging resources. Looks like there's little reason any more to suspect that Directory Opus had anything to do with yesterday's problem. On the contrary, today I have found the image viewer to be super convenient.