Nat'n: I have one of these and fruitlessly tried to get access to the communication protocol, so I ripped out the controller board. Built a very simple PIC based controller that talks directly to the display shift register through the ribbon cable from the controller. The display still uses original LED driver circuity, but the control signals are generated by the PIC.
Took a lot less time to build the PIC controller (course its fixed message right now) that I spent trying to hack the existing board with no success.
Found a link online, thought I'd share it here, runs in windows V/7 on .NET, but source is available and could be ported ... more info here http://www.mahalo.com/answers/im-trying-to-find-a-way-to-hack-mod-the-symon-netbrites and... http:\\www.thepenguinmaster.com