velocity screw

No, you can't.  While you could put a pressure gauge in the opening (if you could find one with the proper threading) it would only tell you the pressure released during the firing cycle.  Furthermore, as your eyes should be on your target when firing, you wouldn't be able to use the gauge under most circumstances.  Related to this is the fact that when you went to check the gauge after firing, chamber pressure would have returned to zero and that is what you would see on the gauge.  Finally, knowing chamber pressure doesn't help you with knowing velocity all that much for two reasons.  One; you would have to know the correlation between various chamber pressures and velocities (which varies based on marker differences and environmental considerations).  Two; velocity is measured as the ball exits the barrel which means the ball is being chronographed as the chamber pressure is returning to zero.