Thank you kindly. I wish that had turned up when I Googled "koreader damaged screen". (Google really is kind of iffy these days, isn't it?)
I guess it would be asking too much to be able to sacrifice only the corner instead of the entire top 15 mm (or side 5 mm).
ETA: Is there some way to scroll the screen by a line or two instead of advancing to the next page?
ETA2: For reference, I opened .adds\koreader\frontend\device\kobo\device.lua and at the end of the "KoboTrilogy" section added the line
Code:
viewport = Geom:new{x=33, y=0, w=567, h=800},
(and managed to bork it up several times by forgetting the comma. Ha ha!)
On an unrelated note, I am apparently running version v2015.11-1800-g0924b57_2018-07-29, via KSM. Is it safe to update?