Adding logging is possible on 2.x hardware – it AtMega644 has enough resources to add it and the board has SD card slot.
I’m running my own modified version of OpenSprinkler software on a regular Arduino Mega hardware, and I added SD card-based logging. When I added it in, SD card support consumed additional 1.2K of RAM, but Mega has enough resources for it. OpenSprinkler 2.x hardware has sufficient RAM as well.
Unfortunately my modified software version will not work directly on the original OpenSprinkler hardware due to different Ethernet controller – I’m using standard WizNet5100 while OS uses a different one. But the same mod (adding logging) can be done on a regular OS version as well.