It is really irritating that Apple don't offer 256GB iMac Pros with the current 64GB LR-DIMM prices (and it being officially supported in Intel), and even the current XNU kernel can do 252GB.
While not a true solution because you need to break open your computer, the newest iMac Pro has socketed DDR4. iFixIt’s teardown[0] suggests the max it’ll support is 4x32 GB for a total of 128 GB.
