Funnily enough I was thinking the exact same thing while poking around my Allroad. Then I read up on the system and decided coil springs were the way ahead
I've seen a couple done with aftermarket kit. Its not that hard really - the front struts are the same design as A4 and A6, albeit taller. The rears are perfectly suited to take a standalone bolt-in bag, and are much the same design as C5, C6, or mk4 rears.
If you wanted OE parts you could almost certainly use the front struts from an Allroad. They are taller than the other C5 models so likely would suit the D2 setup. I think the D3 units are too large in most dimensions, although haven't actually measured one. They just look bigger.
D3 rears wouldn't work as that has the spring and damper combined in to a strut whereas the D2 has them separate. The D2 body is not reinforced in the right places to switch to a strut either.
However... the C5 A6 FWD actually had the option of air on the rear beam for self-levelling, and that uses the same style separate spring / damper setup as the D2. The C5 spring is shorter than the D2, but you could easily make some aluminium collars to adapt it to suit. The only downside is cost as you'd not find those secondhand so would need to buy new.
Regarding control, I would immediately discount trying to integrate factory electronics. The Allroad system is complicated enough, connected to pretty much every drivetrain system in the car, but its a blunt instrument compared to the quantum-physics grade complexity of the D3 system. I would suggest aftermarket control is the only practical way to go with that, integrating an OE pump and reservoir, possibly also valve block, with a dedicated & standalone control unit hidden in the armrest. The Allroad button won't fit a D2 dash btw.