Ideally, the EFI on a motherboard should have an EDID override capability, possibly using a variable stored in NVRAM. I would first try searching a BIOS update file to see if it includes the EFI_EDID_OVERRIDE_PROTOCOL_GUID anywhere. That might at least indicate that it gets used somewhere. I looked at my BIOS update file "Z170XG7.F8" using EFITool.app and searched for the EFI_EDID_OVERRIDE_PROTOCOL_GUID 48ecb431fb7245c0a922f458fe040bd5 which was found in the CsmVideo driver. To test if that driver is used by onboard graphics, pipe output from the "drivers" command to a "drivers.txt" file on the EFI partition using EFI Shell. The CsmVideo driver will have GUID 29CF55F8-B675-4F5D-8F2F-B87A3ECFD063 in the drivers output. If that driver is used by onboard graphics, then there's a chance that adding a EFI_EDID_OVERRIDE_PROTOCOL driver in the Clover folder would work. If not, then adding a driver to the BIOS itself could work (but that's more dangerous).