Eh, I've used both types of pressure portafilters (flow vs. static) and the readings aren't that different. Usually they differ by 0.5 bar with the static ones reading slightly higher. You will get nearly the same reading simply by not locking in the portafilter tightly so it drips a little.
That said, unless you're calibrating equipment to an agreed-upon standard, I don't see any value in knowing the precise brew pressure. If the goal is better espresso, it's more useful to treat the gauge reading as X and then compare tastes/textures when bracketing the pressure at X, X+1, X+2, X-1, and X-2; you'll learn a lot about if and how brew pressure affects your espresso.
If it's just a matter of curiosity, I would try removing that closure nut on the portafilter and replacing it with a coupling/needle valve combo. Don't be disappointed if the reading is nearly the same as the static flow reading.