It is very very difficult with any color film to increase contrast without messing the color quality. You get crossover. Even the Sulfite trick causes some crossover. I can suggest some methods such as adding more CD4 and adjusting the pH back to normal, or just raising the pH with base. But, sadly, these do cause problems.
The best way to do what you want is a simple push, and even that is mediocre IMHO. Remember, I used to coat these films so I know what the problems are in every detail.
Hey, if you can get some Gold, or the consumer equivalent, that has higher contrast than Portra. It pushes no better but starts higher by about 0.01 in contrast (or thereabouts). It is made for the consumer market with higher contrast to give the pix a higher snap and to compensate for the flare of non-professional systems.