well, as cyrille suggested: using no borders, an offset of 0+0 and a dimension of whateve should be almost as good as "fullscreen"
This is indeed how 'fullscreen' works on OSX. Although I initially did this to get around a change in the Apple GL implementation, it might be worthwhile to do this for all platforms. All the code does is ask for the display coordinates of each attached device, number them starting from the main display and then compute the size and position for the window to cover that display completely. I don't know how easy or possible this is on Windows and Linux, but it does enable the use of any display device and also multiple GEM instances to have a 'fullscreen' as well.
cgc