The updater program source code is actually in the Github:
https://github.com/rayshobby/opensprinkler/tree/master/OpenSprinkler%20Controller/software/osFirmwareUpdater
If you look at each os folder, there are lib and source sub-folders. The source code is the same in each os folder. The program is compiled in Processing with G4P libarry for GUI. It makes an external call to the ‘avrdude’ program which uploads the fimrware. The main reason to use Processing is that it can easily compile and export the program to all major OS. The program is pretty rudimentary and there are lots of improvements to be made. If you are interested, you can try to install Processing and get the program compiled. That would be a good starting point to make modifications to it. Thanks.