So I would imagine this would look like:

Demand side
  Splitter
    N-number of branches in parallel, each of which contain a pump and a heat exchanger for each zone
  Mixer

Supply side
  Splitter
    M-number of branches in parallel, each of which contain a pump and a cooling tower
  Mixer

Since this would employ pumping on the demand side, the plant loop will need to be told what type of control to use, and there are some restrictions applied to pumping when you do branch pumping with primary/secondary systems. But it should get you close, and should allow you to control the supply setpoint as with simpler plant configurations, and without the need to workaround with a separate plant loop.

Is there a reason this wouldn't work for you?