Is it possible to have access to "rho" locally? I will be keen of evaluating mass flow passing through a faces for example.
Assuming ideal gas law you'll need to create a new Result (field) according to:
rho = p / (R * T), where p = pressure, T = temperature, and R = specific gas constant, e.g., Rair = 287.04
You'll also have to set the File->Properties, Units->Temperature = K, and Origin = 0.