Thanks for your help, Joachim! My project is school curriculum, with lots of "fill in the blank" exercises. Underscores are too light, and in certain fonts (required for the material) they aren't continuous. Besides, I'm copy/pasting lots of existing exercises from LibreOffice files, and I'm not keen to redo all the blanks. Maybe I'll just have to, though. I tried to use PPX9 instead, but that's even worse, with the underlining appearing/disappearing from view at random.
One workaround I discovered: set tabs to a narrow length and use regular Ctrl+U underlining. Works great, but not for copy/pasting existing work.
Continuous underlining seems like such a basic text function, I can't understand why it doesn't just work like LO. Anyway, thanks for your help!