Thanks to @JorgeB for pointing me to the controller. For those that might have one still chugging away, what looks like is happening is that under load the Marvell-based (AOC-SASLP-MV-8 in this case) controller will freak out over IRQ16 (or 13, sometimes) and crash the server. The disk being rebuilt wasn't even on the controller, but you need all the disks to participate so...
Anyway, so I went and found a different post (also featuring @JorgeB and @saarg in starring roles) that recommended disabling IOMMU by appending "iommu=pt" to syslinux.cfg.
And now my drive is rebuilt. I have some more LSIs on order to replace so I can have IOMMU back, but this works for now!
