I am working on the next release version that brings the protocol up to snuff with firmata 2.5.
https://github.com/DomAmato/openFrameworks/tree/ofArduino_Updates/libs/openFrameworks/communication
This should enable you to use every pin on the mega and includes updated features like i2c, onewire, stepper, and encoder support. Note that some of those features haven't been tested as much as others but all core functionality has been tested
