More questions then answers;) What do you mean by "BLE dongle"? What is running on "NRF51822 device" which you want to upgrade? In general: if your target supports OTA DFU then it doeson't matter which devices is doing it. However depending on what OTA DFU mechanism your target supports you need to design FW or upper layer APP on that magic devices which conducts the upgrade.
I will try to give you all the details I know (I am just starting to learn the device).
The product has two processors the Nordic NRF51822 and stm32l1 Arm Cortex processors.
The Firmware upgrade to the stm32 processor is done by the NRF51822 processor in UART bootloader mode off the stm32 processor I want to use the BLE dongle that is used for regular operation of the system also for firmware upgrade.
Unfortunately this is almost useless info because nRF5x devices (unlike others e.g. STM32) have no default bootloader or ROM. So each FW can implement firmware upgrade differently and without knowing exactly what your device + FW support you will hardly hack it. Then there are indeed "hard" ways like dumping nRF51 flash and reverse-engineering the FW (there is bug in read-back protection so this is feasible even for unequipped "hacker") but I guess this is not what you want to discuss here...
