In my case, I don't need any extra lead to become negative. The tank I dive (and bp/w) is enough, even at reserve pressure. So, if I wanted to become positive I'd have to ditch the rig (or swim). I'm not sure exactly how negative it is at reserve, but negative enough that I can sink to the bottom of an 6' deep swimming pool.
I'd have to switch to a lighter tank to need lead. I guess maybe switching to a soft "positive" bcd might do the trick. I've tried both aluminum and kydex backplates. Still negative with either of those. I do use an STA, so that contributes. Maybe I need to look into one of the plates that doesn't need an STA.