I have grabbed sdbus.sys + dumpsd.sys + sdbus.inf from Windows 8 build 7880, then I patched security cookie and ntoskrnl.exe import to ntoskrn8.sys on needed files. No missing imports at all. This is driver for PCI\CC_080500 + PCI\CC_080501 as SDA Standard Compliant SD Host Controller.
And from Windows 8 build 8056 sdstor.sys + sdstor.inf, then I patched security cookie and ntoskrnl.exe import to ntoskrn.sys. No missing imports. This is driver for SD\CLASS_STORAGE + SD\CLASS_MMC as SD Storage Class Controller
Drivers are unsigned and untested and only for x64 now. INFs are modded and install ntoskrn8.sys automatically. Try to integrate them with force unsigned switch (or NTLite) to boot.wim and let's see how it goes.
https://www.mediafire.com/file/5a58l01bmwc0grn/Generic_MMC_x64.7z/file