Looks like either the regulator or the sensor is going bad on my driver’s side HPFP - anyone know if this is something I can service or do I need to replace the whole pump?
Usually with that code the pump is dying. What engine do you have? Usually when one HPFP dies then the other is not far behind.
that’s what I was afraid of… 
5.2. Are the V8 HPFP’s compatible? I’ve got a friend that’s going to junk an A6 - would be a way cheaper alternative. The V6, V8 and V10 pumps look visually similar, but they’re different part numbers; are they just calibrated for different pressures or are there significant internal differences that make them not interchangeable?
No idea.
I stumped Jimmy?!? 
I think this is what happened when my fuel pumps started dying. Best bet would be to replace them both at the same time and maybe keep the one that didn’t fail (yet) as a backup.
From my understanding the pumps are different. I think the only close pumps are the RS4 pumps but I still think those have a different internial flow rate.
You don’t want to spend the money on the pumps from the V6 only to have them fail. You can try contacting JHM to see if they have a rebuild kit for them. I know they upgrade pumps for the RS4 and A4. They might have something.
bought a new one - local audi dealer was the cheapest by $100 - go figure. 10min to replace and she’s happy again Hopefully the other one doesn’t die. At least I know where to go to get it now.