Thank you @peekay123 for the valuable reply. Bluz however use BLE. We were able to modify the bootloader of the particle photon firmware and dfu a blink LED app and it worked although it had dependency on Wiced stack which has DCT related functions like dct read/write. We are including that library at the moment to generate the executable and to avoid unresolved link errors. We plan on to exclude the library later and have a file with functions dct read/write.
Our next step is to modify system part 1 which is wi-fi related and replace it with STM32F207's Ethernet. We believe we ought to make some modifications as per "Core firmware" since photon inlcudes WICED resources. Could we initialise the Ethernet in HAL core functions so that the board connects to Router which acquires IP from DHCP and use spark cloud wiring functions.
Like you mentioned its the biggest challenge and need thorough analysis of the software architecture/design and code but its doable isn't it ? Appreciate your feedback.