Well, it looks like we're probably not using a PCB for this, or at least not primarily using a PCB. There will probably be a few small PCBs for specific instrumentation and devices along with one piece of structural FR-4 with "traces" made of like 25-mil copper sheet.
100A is a lot to ask of a PCB, I suppose.
apparently sometime in the last week, pcbway (a cheap pcb fab house) expanded their offerings to include no less than _six-ounce_ copper for both external and internal layers... and it's still not really enough because thermal management is Hard