>> GPU accelerated video decoding is again another matter than video
>> decoding done by dedicated video decoding cores. AFAIK, current
>> devices with Cortex-A9 core(s) do it the latter way, not the former.

> Interesting. I had assumed they would use the GPU.

I assume from the block diagrams of various chips with Cortex-A9 core(s) where it can be seen that the 2D/3D GPUs are distinct from the video decoding/encoding units:

http://www.freescale.com/files/graphic/block_diagram/IMX6Q_BD_IMG.jpg
http://www.samsung.com/global/business/semiconductor/file/product/Exynos4210-0-0.jpg
http://focus.ti.com/en/graphics/wtbu/OMAP_44x.jpg
http://www.stericsson.com/images/block_diagram/U9500.jpg
http://www.nufront.com/UploadFiles/image/%E6%97%A0%E6%A0%87%E9%A2%98.bmp

> If the documentation for this function and existing Android or Linux
> code examples exists, why couldn't the guys use this functionality.

If the premises you mention are met, then I think the MorphOS Team could provide support for the dedicated video decoding cores that are inside the ARM SoCs. It'd mainly be a matter of human resources I think, so a hypothetical MorphOS running on Cortex-A9 may (initially) very well lack support for several on-chip functionalities such as video decoding or 3D graphics.