Hi (again),
I appear to be making some progress. After putxing about with tvservice, I've managed to get the following. Only problem is that the screen stays black for a while and the tv's firmware kicks in reports "retrieving data". I think the problem is mode 81 wants 60Hz and TV service -s is reporting 59Hz.
LibreELEC (official): 9.1.002 (RPi4.arm)
LibreELEC:~ # tvservice -s
state 0xa [HDMI CUSTOM RGB full 16:9], 1366x768 @ 59.00Hz, progressive
LibreELEC:~ # tvservice -m CEA
Group CEA has 18 modes:
mode 1: 640x480 @ 60Hz 4:3, clock:25MHz progressive
mode 2: 720x480 @ 60Hz 4:3, clock:27MHz progressive
mode 3: 720x480 @ 60Hz 16:9, clock:27MHz progressive
mode 4: 1280x720 @ 60Hz 16:9, clock:74MHz progressive
mode 5: 1920x1080 @ 60Hz 16:9, clock:74MHz interlaced
mode 6: 720x480 @ 60Hz 4:3, clock:27MHz x2 interlaced
mode 7: 720x480 @ 60Hz 16:9, clock:27MHz x2 interlaced
(native) mode 16: 1920x1080 @ 60Hz 16:9, clock:148MHz progressive
mode 17: 720x576 @ 50Hz 4:3, clock:27MHz progressive
mode 18: 720x576 @ 50Hz 16:9, clock:27MHz progressive
mode 19: 1280x720 @ 50Hz 16:9, clock:74MHz progressive
mode 20: 1920x1080 @ 50Hz 16:9, clock:74MHz interlaced
mode 21: 720x576 @ 50Hz 4:3, clock:27MHz x2 interlaced
mode 22: 720x576 @ 50Hz 16:9, clock:27MHz x2 interlaced
mode 31: 1920x1080 @ 50Hz 16:9, clock:148MHz progressive
mode 32: 1920x1080 @ 24Hz 16:9, clock:74MHz progressive
mode 33: 1920x1080 @ 25Hz 16:9, clock:74MHz progressive
mode 34: 1920x1080 @ 30Hz 16:9, clock:74MHz progressive
LibreELEC:~ # tvservice -m DMT
Group DMT has 11 modes:
mode 4: 640x480 @ 60Hz 4:3, clock:25MHz progressive
mode 5: 640x480 @ 72Hz 4:3, clock:31MHz progressive
mode 6: 640x480 @ 75Hz 4:3, clock:31MHz progressive
mode 8: 800x600 @ 56Hz 4:3, clock:36MHz progressive
mode 9: 800x600 @ 60Hz 4:3, clock:40MHz progressive
mode 10: 800x600 @ 72Hz 4:3, clock:50MHz progressive
mode 11: 800x600 @ 75Hz 4:3, clock:49MHz progressive
mode 16: 1024x768 @ 60Hz 4:3, clock:65MHz progressive
mode 17: 1024x768 @ 70Hz 4:3, clock:75MHz progressive
mode 18: 1024x768 @ 75Hz 4:3, clock:78MHz progressive
(prefer) mode 81: 1366x768 @ 60Hz 16:9, clock:85MHz progressive
LibreELEC:~ #