Peeking into the Hardware (disassembling the SRL2 main unit is really easy since its only 4 screws holding it together) helped me finding out it being a CSR device.

Understanding the outer firmware file format was easy after spotting the pattern in the header. Each file inside is MD5-summed and stored with offset and length in the header.

Split Layout Headsets got a DFU file for the internal flash and a vp.bin for the external flash. Only those can be modified so far since (found out later) the external flash is not signature-checked.

Extraction of the external IMG is possible with the ADK toolkits since its a Filesystem image in their format.

Current workflows are written into the linked Github code