Actually, I have Teclast C260 Clone, and it supports PlayFX. It depends on the firmware. My original polish firmware doesnt have it, like many other functions (rkp is not supported), but firmware from Teclast works, and PlayFX also.
I too think that if we had access to the system functions we could do even better music player than in 2706. Capabilities are huge, but companies are very lazy at creating firmwares. They just want to sell it as soon as possible. It's all about the money. For example if they released the source of those libraries it would mean that even Teclast C260 would support videos without converting with subtitles, OGG format, Album Art, SWF player, linux and such. Such player would be worth much more for little money. But its not what companies want, to give these things to us, they just release a new player, with almost the same hardware and just better firmware. Cash cash cash.
Sometimes I regret I am not a hacker to just hack into their computer and steal the source code.
In terms of hardware, it has huge capabilities anyway. For example RK260X supports bluetooth!! It supports USB fully, so it would be even possible to connect some devices to the player like pendrive, printer or webcam.
Is there any way to disassemble rfw firmware? Maybe RKP dissassembler could be modified to handle rfw files, I mean the type of code of rkp and rfw is exactly the same, only the header and resources are different and in different offsets.