Sounds like you are just trying to set an outlet boundary reservior with pressure of atm or 0.528*P0. Not sure if this will work, but may worth a try:
Use a reservoir at 1 atm, and conncet a controlled pressure source before that. The input to the pressure source can be the output of a look-up table or some function that will transit from 0 to 0.528*P0 rather quickly. You need a way to sense and calculate P0 of course, which means you will need to make quantities like rho and vel available by modifying some of the existing blocks (or maybe you are already writing your own custom blocks).
