The disaster system I mentioned earlier actually had the neutral temperature loop that Chris mentions.
Unfortunately for that system, the building was all exterior, so all of the heat input to the loop was from a massive central hot water generator, running at ridiculously low temperatures, and short cyling. The low temps caused condensation of the flue gas, and as the equipment was not designed as condensing equipment - well it aged prematurely.
I recommended converting all of the heat pumps to DX cooling only, adding hot water coils for heating, and upsizing the fans accordingly. Even with the necessary replacement of the boiler, payback was under 10 years.
The other problem with this type of system in general is that the "neutral" temperature is off of the best efficiency point for both the condenser and evaporator side of the heat pumps.
The whole thing is thing is just terribly inefficient, and uneconomical unless the utility rate structure is unusual.
Might work if you had a local co-gen plant, feeding the waste heat into the neutral loop to supplement the heat from the interior spaces. Probably even harder to obtain the right building layout balance though.