Take all of this with a grain of salt because I have no actual experience with the LIII preamp (though I've installed the Ghost system a few times).
The Ghost has an aux power out but you can't draw too much current from it. I would probably take 9V from the power input to the Ghost. But you have to watch how you have the output jacks wired up, so the circuit is completed (and you get power) when you plug something into either output jack (assuming you use a separate preamp for each output).
So ... to do what you want, disconnect the mags from the input of the Ghost preamp and instead wire the output of the pickup selector (what would normally go to the volume pot) to the LIII preamp (basically follow that schematic, removing the volume pot as well, etc.). Then, wire the output of that preamp to the other output jack. You won't have a blended signal, but you'll get buffered/boosted mags at one jack and buffered piezo at the other. (You won't have a passive output, though you could wire something up for that.)
In terms of pickup configuration, what you're sending to the LIII preamp is the output from the pickup selector switch, so the actual pickup configuration shouldn't really matter. I don't know how you have your Silo wired up at the switch, but from what I can see of the LIII (with the 4-pole 5-way), you would change a few wires on the lever to account for volume reduction in the humbucker positions. That's easy to adjust later, though.
Again, I'm just guessing. Doesn't look too difficult, but you might need to play around with it a bit to get it all as you'd like.
