I suppose that zbuffer is implemented into the hardware, so you can't work on modifying the algorythms, except fo the parameters.
I also suppose that you made some tests to decide wich values for far and near plane give the best results in FSO, so it is probably useless to discuss about changing them.
I also suppose that you probably have already looked into w-buffer, to see if it could be useful.
BTW, wouldn't be possible to implement in FSO both 16bit and 24/32bit z-buffer, and let the game switch between the two modes, depending by the video mode choosen with the launcher?
I'm asking this because programs like 3danalyze can force 24 or 16 bit z-buffer (it also can force wbuffer over zbuffer, and viceversa), but only if supported by both the card and the game.
If this can't be possible, then you should go to scratch support for older videocards (maybe using a "safemode" to disable most features and let people with outdated systems to still be able to play the game?), since this could limit a bit our possibilities, for example to add true planets, semi realistic landscapes, giant ships (who said death star?), and the like