Just for fun, I also confirmed that I can output 15kHz video and get around 832x288 pixels. I tried interlaced mode but it is flickering and uncomfortable.
Also this is pushing the timings a bit, you can see the scanlines getting noticeably wider accross the screen, not sure how to improve thatbut I can tweak the timings further.
Also, uboot doesn't have a 8x8px font, that would be helpful here. And I didn't manage to enable the 4x6px one.