Hey guys!
I've done some research about one rk2607 player I have (
link). It's indeed an dual core solution - ARM7EJ-S main processor (according to firmware sources - up to 270MHz, but it's not a limit for hacking) and a ZSP400 architecture DSP.
What I've found that in /appdata folder in hidden partition there is a file named "base.rkw", which contains ARM code. If I delete that file, the player doesn't boot up. So it definitely contains a program that gets executed right after boot up (player doesn't do any initializations of any devices (for example, screen) without that file).The only information which is needed for hacking, where and how exactly does base.rkw run. The beginning of disassemble of base.rkw looks like a interrupt vector table, therefore it's possible that it's just loaded to the beginning of RAM and all interrupts are headed there. Anyway, it's not a hard task to place a custom bootloader that configures processor in our way instead of base.rkw. So I think it's green light to port ucLinux into rk2607 as all device drivers are already written in the code that came with SDK.
P.S.
Other rk2607 (un)features that should be mentioned:
no MMU (only ucLinux can be ran, definitely no X11)
USB host
no RTC