I went through this myself last year. I wanted to add weight. The screw is strange in that they are glued in, and have a wide, thick slot, that normal screwdrivers won't fit in well, and in the end will just chew up.
I went to my local discount goods store ("Ollies") and found a medium-large pry bar that had a straight-tapered end. It kind of looked like a very large flat blade driver. However where the end flared out before the
blade was too wide to fit in the end of the cue butt. So I used a grinder to grind down the width to the right size.
At this point it fit the end of the butt, and the slot in the screw perfectly, but I could not turn the screw. I had read about heating the driver, but I didn't want to de-laminate any of the surrounding wood, so I tightened
the now large screwdriver handle in my vise, and then used a jar-opening rubber around the cue butt and turned it with all my might. It finally broke free.
Once I had the original bolt out, I matched the thread with a heavier, Hex-Head bolt from the hardware. (3/8th I think) I took the bolt home, and ground down the six edges of the bolt head to make it round, tapered
the head of the bolt into a cone, and the using a cutting wheel, cut a slot into the head. Now I could screw this one in the same as the original. All in all I think it added around an ounce.
Good luck, hope this helped. If you need anything further, feel free to send me a private message.