I don’t think you can set the quality directly:
http://forum.inductiveautomation.com/t/forcing-the-quality-of-a-tag-using-python-script/5565/2?u=witman
Another option, though I don’t particularly like it, would be to create an expression tag linked to the value of Pressure_PV when Pressure_WB is good, but otherwise evaluating to something that results in bad quality.