I would like to confirm that the above video settings work for me too on a samsung laptop pannel 1280x800 using a tsum v29 universal lcd controller, BUT i had to set PLL_VIDEO=330 to get correct colors. Using 336 or from 332 to 338 results in blurry mostly green screen. PLL 330 nad 331 worked for me so i havent tested anything else.
P.S. :
I tested first option and it works too whithout changing pll using the stated 334.
I made a patch for those that want to try it but dont know how and where to add the modlines. To activate resolution change screen0_output_mode = 38 screen1_output_mode = 38 and pll_video = 334 in script.bin
Or add :
38|1280x800) # res
HDMIMode=38
pll_video=334
;;
in /usr/bin/h3disp and use h3disp to set changes.
If you want to risc it and trust me you can try my precompiled packages for OrangePi_PC.
https://drive.google.com/drive/folders/0B40z5IEobXstb0g0am9NMHkwWTQ?usp=sharing
z-0004-add_1280x800.patch
And a big THANK YOU to mariuszb for finding the correct values for this resolution.