After no luck on changing DSP, I began trying other tools from bluesuite. BlueFlash came up.

It has buttons that supposedly do what I need. But greyed out, it says "processor running". As soon as I clicked Stop Processor, it spew out an error, because the chip immediately loses power.

Using same trick to force the power to the chip again, now I got lucky. I stopped the processor and dumped the firmware, and even verified it. You can find it in project files.