Thnx repu1sion!
These are some really good leads! I have tried them, but...
..sadly it does not work. I'm slowly but surely loosing my mind

Ok, here's what i have been up to:
- uEnv.txt does not work . I dont know why, but when I set video mode in uEnv.txt, U-boot just ignores it and loads 1280x720, as per script. Bummer. Solution: change kernel line and run saveenv, reboot. Kernel line worked, LCD not. I tried stock KitKat and Lolipop images from the Wand site. Both did not work with kernel line mod.
- i have hacked the kernel, as you described. Works on desktop LCD - it enters 1280x800 mode. My display does not

Ok, so here's the problem: i have set up the enviroment for KitKat (4.2.2_1). After hacking it still did not work (but on desktop LCD worked), so i decided to go low-level. On my panel i have TIs TFP403 (HDMI to RGB) and there is a funny thing going on: the HSYNC signal is missing!
When i run the Lolipop image, i see HSYNC, when i build KitKat (or run stock image), the HSYNC is gone! In U-boot i can see HSYNC, but as soon as i enter kernel, it does not work. Strange.
I guess i will setup the Lolipop enviroment and try your hack there. Any other suggestions?
I also noticed that you have set only FB_SYNC_VERT_HIGH_ACT and not FB_SYNC_HORIZ_HIGH_ACT. I know that some displays do not need HSYNC, if they have VSYNC and DE. That would explain why desktop LCD is working. Is it possible that your display does not use HSYNC?
Anyway, am I missing something? Is there some flag in HDMI driver to enable/disable HSYNC?? Is that even possible? A real nightmare this is. I will never assume that using HDMI over LVDS or RGB is better

Best regards,
Matej