[Jon Stratis]

Stick a T on the input to the tank. One side of the T will be your input, and one side will be your output.

Air does not need to flow through the tank from one side to the other. Generally speaking, air flows anywhere it can, from high pressure to low pressure. The tank allows you to store high pressure air, but that air could flow back towards the compressor, if there was lower pressure available there (for example, if your emergency relief valve releases). With some minor variations, you'll have the same pressure everywhere on your high pressure side at any given point in time, and that will flow, as appropriate, through your regulator to the low pressure side when needed.