I'm one of the maintainers of the opensource project https://github.com/mirko/SonOTA , however with the latest release there appears to be certificate pinning (or similar) happening. This means that users can only upgrade the firmware in the Sonoff devices if they open it up, and solder in headers.
Using this OTA method users could get all of the benefits of an open platform, without the the need to open up the device and solder etc.
i full support this request
Also from here a full support to be able to flash custom firmware without opening the device.