Here's a closeup of the main board. There isn't much to see, because it is obscured by copper shielding foil and a speaker assembly.
More can be seen after removing the foil and speaker assembly. However, it's not very interesting. That's just the data flash and 2 MB of SDRAM. The SoC running it all must be hiding on the other side.
Detaching the circuit board is easy. However, the SoC is still hidden behind the LCD support.
After carefully removing the LCD support, I can see that the SoC is the Telechips TCC760. The datasheet is available!
Here's the TEA5767 FM radio:
This is the CS42L51 CODEC. To the left, R59 through R60 are 47 kΩ pulldown resistors which set the TCC760 BM (boot mode) bits to 000, meaning NOR boot without encryption. Making the right side of R60 high at reset time will change BM to 010 and select USB boot instead.
This can also be done on the other side. The capacitor pin closest to the speaker connector is +2.5V and the bottom short trace near the corner of the foil is BM1. I'm using a resistor rather than a short because the same pin is used for LRCLK for the CODEC. Yeah, it's messy, but it's temporary. I'd like to install a switch after I figure out how to run code.
Here's what shows up in Device Manager:
It might be possible to use the Rockbox tcctool program to upload code. A single one line change adding the device to the device list enables uploading, but I have not been able to confirm that any uploaded code actually runs.
The USB device ID is supported by the TeleChips firmware download driver vtcdrv.sys found in iAUDIO_COWON_D3_Upgrade_V4.53GL. There is also a VtcUsbPort.dll in the upgrader. Using these would require reverse-engineering the API. I'm hoping a simple change to tcctool will make this work.