Unfortunately this will only be interesting to you if you buy one of these keyboards, but this morning, it only took me an hour or two to put together a Python Logitech G15 keyboard daemon, which replicates most of the functionality from the included Windows drivers. It also provides a really simple Python API for hacking the display.
% cd g15lcd-1.2-pre0
% tar xvjf .../pyg15.tar.bz2
% python run.py
To try it, you will need both Python and Twisted.