According to the Guide to the Macintosh Family Hardware, Chapter 4 on VIAs, various machines use bits of the one or both VIAs to determine what machine they're running on. For instance, on the II and IIx, bit 6 of VIA1 data register A indicates a II if low, a IIx if high. I've only got the 2nd edition of the book, so it doesn't cover the IIsi... But anyway, if you've got the documentation for the IIsi, you should be able to figure out which bits (and hopefully the subsequent pins) that provide the machine identification. The thing is, they don't provide the full definition of the machine. Presumably the software can determine roughly what class machine it is running on from other aspects, and only use the CPU ID bits of the VIAs to provide disambiguation when it's otherwise too difficult to determine. For example, you can't use VIA pins to convince anything you've got a IIfx when you've really got a IIx. But you could probably convince something you've got a II.

But, while reading that chapter, I came across this, documenting the functions of VIA2 on the SE/30/II/IIx/IIcx:

It provides a signal (/POWEROFF) that shuts off the power supply. (In the Macintosh SE/30, this signal goes to the processor-direct slot only. An expansion card can use this signal to determine that it is about to lose power.)

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Then later in the section about VIA2 data register B:

When the v2PowerOff bit is set to 0, the power supply is shut off in the Macintosh II-family computers. The signal controlled by this bit is available at the processor-direct slot in the Macintosh SE/30 so that an expansion card can use it to shut down operations before losing power from the computer.

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That seems like the SE/30 should still be toggling that line even though it's not a soft-power machine, which means my INIT shouldn't be necessary at all.