Not sure if you're still curious, Ross, but it looks like the recipe is in here. The only thing I see missing which may or may not be a concern is the all-steel tcase. If memory serves, there's already a guy down under who likes to mess around RWD in his E39A and he specified the all-steel tcase.
1. Weld the center diff (or install a spool, I've seen these available)
2. Disassemble the front axles, insert stubs into gearbox and hubs, discard actual axle
3? Route power through all-steel transfer box
You guys get to ditch the restrictor if you go 2WD? Production classes up here run a 32mm restrictor, open AWD runs 34mm, but 2WD goes wide open.
Hope all is well at your end, mate. Don't be a stranger.