I submitted
the bug and got a quick response: Wontfix with no explanation.
I'm disappointed, but not surprised. After decades of working in the computer industry, I'm used to accessibility issues being ignored.
I took a quick look at the Qt documentation and the setCursorWidth(int) function is there for text editing windows (specified relatively high up the inheritance tree). However, when looking quickly through the Sigil code, I gave up trying to find the specific bit that handles the editor to see what would be involved.