If I needed the regulator output to go to zero volts when disabled, then I would need to look at using opamps with rail-to-rail input/output. The AD797 gets me under 2V (with the 2 samples I have in the regulators I get to 1.8V in the positive regulator and -1.5V in the negative) which are about where the datasheets say the limits of the common mode inputs are supposed to be. My reason for wanting to enable/disable the outputs in a timely manner is related to a cautionary note in the AK4499EX literature that states damage to the DAC outputs could result if the analog 5V power inputs are removed before or applied after the +/-15V I/V opamp power is removed or applied.
My +/-15V for the opamps is derived from a dedicated torroid thru a pair of LM317 that produce +/-18V (pre-regulator) then to the Jung-Didden super-regulator.
The rest of the power is from a dedicated torroid that produces a raw voltage for the DAC circuits and a raw voltage (from the other secondary) for the USB to I2S controller and Arduino display controller thru switching regulators that have enable/disable controls in order to allow me to hold off startup of the USB and display micros until the DAC is up. For the DAC board,I use 3 stages of regulation for the analog 5V. First, a switching regulator (mostly to sallow me to control the enable/disable, if need be) which is set at 15V, then a LM317 set at 9V. That is sent to the DAC board where four separate "analog side" regulators produce 5V for the analog portion of the 4499 and two 5V retgulators that feed five 1.2, 1.8 and 3.3 volt regulators for the digital portion of the 4499, plus the AK4191 and a microcontroller used to set parameters of the 4497 and 4191.
Both torroids are powered on thru the same switch, a nd obviously power off thru that or when power is removed from the unit externally.
I have a fairly simple timing circuit that controls when each power supply is enabled after AC power on and issues a disable whenever AC power is detected to have been removed. That is the main purpose of the switching regulators. There is no switching regulatore in the power path of the +/-15V supply and given the level of power filtering and dual regulation, the +/-15V is slow to turn on compared to the 5V :analog" voltage, but slow to turn off as well. So being able to disable - or get the +/-15V under 5V before the "analog" 5V drops out is the goal of this exercise. Getting it quickly to under 2V works very well with the method you suggested. Under normal power up circumstances, in my unit, the 9V supply will already have booted up and removed the short around the LM329 before the super-regulator even gets usable input from the pre-regulator. It is the disable/shutdown part that really needed the quick response, and I have that now.