@jannis: concerning your last question, i don’t really get the point of the relation between digital/analog in!?
For the arduino(mega) this should be fine connecting them. What you would most likely need to do, is to check the StandardFirmata for the Mega, if it supports the configuration of all those pins - which i guess. What i can (sadly) asure you is, that the current implementation of the FirmataPlugin is maxxed to 2 ports on the board. That microcontroller speak means max of 14 digital inputs. Simply put: even the Mega will only have 14 usable digital ins.
As i am coding on the plugin at the moment, i am happy to pull in the extension for more ports :)
Concerning the ATMega chip even on the standard Arduinos, they can be configured to have a total of 18 digital inputs (14 digital - 2 for Rx/Tx + 6 AnalogIn).
All in all, i suggest going for the IOWarrior, if you do not have an Arduino handy; though i don’t have any experience with the Warrior…
…just don’t forget the resistors…