From what I can tell, the xDuoo D3 is based on an Rockchips RKNano-D SoC. It has a pair of asymmetrical Cortex-M3 cores (150MHz and ~650K of RAM for one, 300MHz, 384K of RAM, and some HW accelerators for the other) though both appear to be able to directly access each others' address space. Code typically executes from flash
Given the RAM limitations and the convoluted internal architecture, a straight port of Rockbox is probably not realistic, but it is conceivable that the rockbox codebase could be restructured to work well on this thing. However, given the utter lack of documentation on this SoC and that convoluted architecture, this would be a reverse-engineering nightmare.
So, tl;dr: Don't expect rockbox to ever get ported to the Nano D3.