It works now.
Turns out it was the disks. Before I bought a bunch of disks for it I was trying it with some old disks I had around figuring, "fuck, SATA's SATA, right?". Turns out old SATA disks can't do hot-plug? Maybe? But even cold plug I think failed? Maybe multiple of my old disks are bad? Who knows.
But I bought a new 500 GB disk today after a dude on a mailing list said that it worked for him, and sure enough... no problem. Disk works hot-plugging between either direct or port-multiplier bays.
So then I went back to buy 4 more but they were out.