I would like to see a Pic of it. There are '040 adapters for the IIsi, so a PowerPC accelerator is not too farfetched. The issues I see is its drawing it's clock signal from the IIsi, thus making for a slow 601 like the Apple version of the board did.

And I remember long ago, one of my students having a 601 accelerator for his LC 475. It was not the fasted thing but it gave him PowerPC capability in an LC machine, and the LC's PSU is much weaker than the IIsi.