not only it's possible but it makes me thinking about something....
If the emulator is fast enough to emulate two or more gba on the same pc, it's probably possible to program a trick to enable multiplayer through a network:
all PC that are joining the game must emulate all the gba (the emu don't need to display the screen of each of them but it still has to emulate all of them), then you only have to use kaillera to send the control the players are using to all PCs...
it's probably slow and hard to implement but at least it's possible