I don't have the setup yet, but from what I recall, nitrogen is not very soluable in water. By storing and serving it at 30-35 psi, you are essentially allowing your restricter plate to work properly, while not over carbing the beer when using beer gas (ie, CO2 pressure is only around 10 psi of the 35).
So you could just hook it up to the beer gas and force carbonate it like usual. However, since (I assume) beer gas is more expensive - you might as well just use CO2 at 10 psi to carbonate - then switch to beer gas at the elevated pressure to serve.
I could be wrong, but that is how I understood it.
Mylo

