Alright, I’ve figured it out, based on this post. The magic is:
pcie_ports=native pci=assign-busses,hpbussize=0x33,realloc,hpmmiosize=128M,hpmmioprefsize=16G
With this^^^ on the kernel command line, I can just plug in the eGPU an it works, no problem at all. The nvidia kernel module loads correctly and I’m calculating Folding@Home on the eGPU right now, so it definitely works.
(My machine won’t boot if I add the recommended nocrs to pci=..., because the kernel can’t talk to SATA controllers and drives in that mode and freezes forever while trying to do so. But the eGPU works without nocrs just fine, so I’m not messing with that any further.)