Well, this doesn't necessarily apply to the IIsi, since the IIsi's video seems to kinda sorta replace the functions of the 2nd VIA we're using in the SE/30 to soft power off. As I understand it, the IIsi's video chip provides some level of emulation of the 2nd VIA, but it's not used for soft power off.

The SE/30, IIx, and II used bit banging of the VIAs to do ADB, but the IIsi had a separate ADB controller. According to the IIsi dev notes, it looks like the ADB controller is what controls soft power off in the IIsi:

The power off function is controlled by software. Using the menu command “Shutdown” causes software to send a special command that enables the ADB microcontroller to pull PFW low, causing a power supply shutdown. This gives the Macintosh IIsi computer sufficient time to complete pending activity before AC power is removed.

Click to expand...

So, "maybe" on the IIsi soft poweroff, but this INIT unfortunately won't help...