After a minor glitch (forgot to export the soldermask layers), the first PCBs are now being made at @Seeed Studio.
The backplane has four PCIe 1x slots, plus debug headers.
The modules are just adapters from Raspberry Pi header to my draft PCIe 1x specification. During the process of designing the PCBs I found a few signals were pretty hard to route (I'm only using 2 layers to keep the costs down), so I think I'll re-map some pins and go to a bigger connector for the next version.
PCB designs are not updated in Github yet, I'll do that this weekend.