i'd like to ask, if it is possible to make a movie player for the rs90 cfw, since the stock fw can do this flawlessly?For OpenDingux you could perhaps rely on an unaccelerated version of MPlayer or MPV. It might be able to decode some low resolution streams. The Dingoo A320 and the RS-90 both use ucosII as OS/native firmware and have the same amount of RAM. The Dingoo A320 in the linked video is overclocked, so I wouldn't expect the same performance on an RS-90.
The Dingoo A320 at least had MXUv1 but the RS-90's soc does not even have that...
When i tried Divi-dead on it, i ported the RoQ video player to it and even though its known for being fast to decode, it was still too slow for the RS-90 so i had to disable that... (video's resolution was 240x160, like the RS-90)
Unless you use a format like Motion JPEG, i don't know how you can get decent speed out of it. Not even Cinepak would run smoothly on it unless someone writes a proper decoder for it.