

You can design for ramp shutoff, brick-wall current limit or even 
fold-back cutoff....sounds like you want "BRICK-WALL" current limit.

Your lead is correct to pull down the bias to the series regulator base 
drive. In order to get the brick -wall, you need enough voltage gain on 
the current sensor.  Normally a darlington on the current sensing 
resistor will be adequate, but for infinite gain.. use an op amp to sense 
the current releative to a stable voltage reference and use a driver to 
turn off the base bias on the series-pass transistor.  Phase compensation 
may be necessary with this closed loop control system if yopu have more 
than 180 degrees phase shift in your feedabck circuit at unity gain OK??