In researching this problem, I discovered the command line utility
qlmanage, with a rather simplistic and obtuse man page. Still, doing
qlmanage -m gave me a list of some 460 registrants for QuickLook services!
So, I downloaded the excellent open source editor TextMate, and used the "Text —> Filter Through Command… ⌘I" menu item, and entered
qlmanage -m | sort, and instantly had an editable list of all ~460 QuickLook services.
I then pressed "⌘F" and entered "raw", then "⌘]" to right-indent the line, then "⌘G" to find the next instance of "raw", and repeated the "indent, find next" keystrokes to the end of the file. That narrowed it down to fifteen suspects.
Surprise! Nearly all of them used "/Library/QuickLook/SneakPeek Photo.qlgenerator" as their QuickLook driver! I guess I must have installed it at some point…
But I must have had some reason for installing it, and I didn't want to lose whatever it was doing for me; I just didn't want it to do its thing to Olympus ".ORF" files, because a "naked" MacOS Catalina apparently handles those files just fine.
So I thought I'd poke around further. The driver is actually a "bundle," a directory containing executable code, resources, and an "Info.plist" file that generally holds configuration info, including what sorts of files the bundle operates on.
Looking in "Info.plist", I found this line:
I commented it out, using the XML comment prefix/postfix:
I then went to the terminal, and reset the QuickLook process, using
I then went to the Finder, and selected a ".ORF" file. There was a notable delay of a second or two, and then (be still, my heart!) the icon turned into a miniature version of the image! I pressed the space bar, and the raw image file showed the proper preview, instead of the generic image icon!
As it turns out, my reticence to simply nuke the entire "SneekPeak Photo.qlgenerator" was unfounded. Only after going through the above to selectively disable it for Olympus RAW files, did I try to find it on the web. It used to do neat things for image previews, including showing some image EXIF data and a histogram. The company no longer supports it. Too bad; it was pretty neat! Someone should do something like this!
So, the best thing to do is to simply delete
/Library/QuickLook/SneakPeek Photo.qlgenerator and execute
qlmanage -r, which may improve system stability, since it's thirteen-year-old software, and apparently doesn't do anything useful any more.
I hope this saves someone a bunch of trouble!