Is it possible to boot Windows 1809 system from Storage Spaces
I was able to get a copy of the UEFI firmware for this model, and opening with UEFITool I can see a DXE driver called "MsHyperSpaceDriver" - description is "Storage Spaces EFI Driver".
So that would seem to be how it does it.
I've read of NVMe DXE drivers being injected to UEFI firmware for the purpose of adding boot compatibility, so perhaps its possible to do something similar for this driver.