Actually, there are several mentions of the NuBus SE/30 notion. Bunsen was looking into doing what you just tried.

I said it'd be problematic due to the /NuBus signal being N.C. on the SE/30, but I was mistaken. It's the IIfx where this pin/signal is N.C. which makes perfect sense in light of the following.

The SE/30 PDS has an interesting spin on the signal at C2 where the IIsi has the same /NuBus signal on the connector.

Pin C2 on the IIsi 68030 PDS /NuBus NuBus space address

Pin C2 on the SE/30 68030 PDS /NuBus

Signal descriptions for the In the PDS: machine specific signals
In the Macintosh SE/30, indicates address in the range $6000 0000 to $FFFF FFFF. This signal is active when the CPU addresses the built-in video display. (Expansion cards can use this signal and further decode the slot ranges to avoid conflict with the video logic)

Click to expand...

The IIsi NuBus Adapter has no provision made to "further decode" this signal, because its NuBus PseudoSlot interrupt/memory address range/whatever is fixed at $9 while its Vampire Video PseudoSlot ID is fixed at $0.

So it's no surprise that your Video display wigged out and apparent memory conflicts caused boot failure . . .

. . . but this need not be the case! }

It makes sense that this signal is N.C. on the IIfx PDS, as all six NuBus Slots are already implemented on the MoBo and the Address Mapping handled in ROM.