Nordic has an example of a bootloader that can be used to upgrade the firmware but it is not doing it over the air. We have implemented a bootloader that initializes the SoftDevice stack and provides a firmware upgrade service.
I our case we have an external spi flash where we buffer the firmware image before we write it to internal flash, but as long as you're careful not to write to flash while the radio is active it should be possible to do without an external buffer. Radio notification can be used to see when the radio is active.
