Doing this with a Seagate 2TB SSHD + https://partedmagic.com/ + a USB3.0 drive dock for the old drive. Other than a scare with the LCD temp sensor (the plastic front snapped off, but the pins were still good), this appears to have worked out!
SSDs may not work. Tried two different ones already on one of these (a Crucial and an A-Data), and no luck. Not sure if it has to do with missing BIOS/UEFI options, or a whitelist of parts in said firmware.