If the EAS Rangie Compressors are anything to go by then answer to filling tyres would be NO, not really.
They have a 10L (approx) reservoir, fine for pumping a bicycle tyre or running a locker provided you have a regulator of some sort (set between 80-105 PSI)
4WD Tyres - would take WAAAYYY too long.
http://www.rangerovers.net/outfitting/air/arbair.html
It's old info, but gives a bit of an idea...
The DII's with SLS in the rear didn't even have a reservoir. (AFAIK)
I'd say the D3 would have to have a tank though, having to run 4 corners worth of springs.
But the MAJOR ISSUE with tapping into LR on-board air suspension is the 'dry' air within. Again, I'm not sure about D3's but the EAS Rangies have a regenerative dryer - carbon particles or something that absorb the moisture by air flowing one way which then regenerate by having exhaust (dry) air flow back the other way, taking the collected moisture with it.
In otherwords - it's designed as a sealed system. You have to be careful you don't 'overload' the regenerative dryer by allowing more 'incoming' air than 'outgoing' air.
The small volume required for a locker shouldn't pose a problem though.
I actually ditched my EAS Compressor and set a solenoid valve up in it's place - to feed air in from my new BOSS compressor and 15L tank in the boot.
This way I have a big arse compressor & tank to do tyres (primary circut) that also feeds the EAS & 10L tank - which is on it's seperate circut.
Works good too...
I pulled the old compressor out because I have grand plans to make a pressurised water tank for the camper trailer - but that's another story....
Hardy