RKNanoD is a dual-core Cortex-M3 SoC.
core 0:
16KB boot ROM
64KB PMU SRAM (retained in low power sleep)
320KB IRAM
256KB DRAM
core 1:
128KB IRAM
256KB DRAM
Codec acceleration HW (not full decode!)
Memory is private to each core, and there is no direct communication between them. All in all, this requires quite a different software architecture than rockbox has been written for. A [very-]reduced-feature port would be a ton of work, even if full datasheets were available.