I wonder, does EOMA68-A20 support these commands, especially without proprietary firmware for it's Mali GPU?
This has nothing to do with MALI: a GPU is a (co)processor which lets you do computations often useful for graphics (but nowadays also used for other things), so it is usually wired such that it can write directly onto the framebuffer memory.
But the GPU itself sees the framebuffer as some chunk of memory, writes to it, and that's it. It has nothing to do with bringing this chunk of memory to an LCD or some other display, which is instead done by some other circuitry (called the "display engine" (DE) on the A20). The same goes for the video-decode accelerator (aka VPU), which is usually also a completely independent piece of hardware, BTW, which also just writes to the framebuffer.
In the x86 world, since the graphics cards include both the GPU, the VPU, and the "display engine", those notions are usually conflated, but technically, they are independent, and in the ARM world they tend to be clearly separate.
So, the question in your case is if the DE can handle your timings and if the `xrandr` thingy will be able to properly configure the hardware. AFAIK the A20's hardware should have no trouble with a 1600x1200x85Hz timing, so the question is more on the software side.
For the "vanilla" kernels, I know there's some "sunxi-drm" module available (tho AFAIK it still hasn't been merged into Linus's tree), so some of the infrastructure is available but you still need an Xorg server on top (presumably the `modesetting` driver might do the trick).
Stefan