If you're using a piezo, arco is always going to be a lot louder than pizz. I don't think it makes sense to handicap your arco sound to solve an amplification problem. You should bow (and pluck) whatever way sounds the best and then find a way to amplify it effectively.
What I've been looking into as a solution for this is a high quality active volume pedal of the type used by professional pedal steel guitarists (the most highly regarded ones are by Goodrich or Hilton). Haven't picked a specific one yet, because of the challenge of matching it up to my existing DC power supply; however, my gig pedalboard was stolen a few days ago, so I guess I have more options now once I come up with a replacement.
A good clean compressor or limiter could work too and wouldn't require interacting with the volume pedal while playing, but if I was going to do that I'd want the threshold lower than where it would be triggered mostly by bowing (for the music I play where I need relatively little dynamic range). Also I like the idea of being able to mute or do volume effects without moving out of position.