Hi Pierre,
Your tailpiece, like most of the instruments we make, is not symmetrical itself, and is not centered to the neck laminates, but rather to center of the string spread.
When I look at the picture jazzyvee linked to, the top of the tailpiece seems centered, but the bottom point is a little to the right. When I look at my other pictures, there is some variance in position for the tailpiece. The person setting up the bass decides on the best position for the hardware to make the playability the best possible.
I'm afraid that there isn't much to be done except to build a new tailpiece to fit better, but it still would be unlikely to be perfectly centered, although I agree it can be improved.
Sadly, I think I would need to whole bass here to do the work. Maybe if you are planning a trip to visit San Francisco sometime, we can coordinate and get measurements from your bass and then send the tailpiece to you when the finish work is completed. I placed a note in this instrument's file so that if we do get our hands on it again, we'll know what to do.