Well the SE/30 didnt have a soft PSU, but it did have a power-off signal routed from one of the VIAs to the PDS. So he just made an extension to actually toggle this pin when the machine was chosen to shutdown.
This pin is used in combination with your own circuit to hard-cut the power to the power supply, Still need some sort of a standby supply built to handle it, or use a battery like the Mac IIx. Basically a relay stuck inside the original power supply to cut-off the AC supply to the main power supply unit. This relay would be connected to your own logic circuit powered by either a standby supply, or the PRAM battery. Two wires from this circuit would then go to the ADB power-key, and the VIA pin. It would be a hack for sure, and something is definitely NOT solderless.
For a plus or SE, if there are no extra pins available to a VIA, it would be a fair assumption a 3rd via would need added to add some expandability to the machine for this type of setup. And an extension written to be aware of the new addon circuit.
Although I'm sure you'll have to amplify the on/of signals to trigger the relays)