UEFI EFI_HII_HANDLE (void *) is not defined here; I am just using Handle. Should it be re-defined or re-exported?

I think a new type alias would be good to add, pub type HiiHandle = *mut c_void

UEFI EFI_KEY doesn't explicitly define values; this does. This seems necessary for a C interface. Should it be kept?

Just to make sure I understand correctly, you are saying that the C version relies on implicit enum member numbering (first entry 0, second 1, etc), while the Rust version has explicit = 0, = 1, etc? That's fine I think, we need to be more explicit in Rust. I would remove the "// NOTE: UEFI does not define the values for any keys" comment; UEFI is implicitly defining the numeric values by specifying the values in terms of a C enum, which has defined rules.

Key modifiers are grouped as a type; UEFI uses top-level defines with the suffix _MODIFIER. Should it exactly match UEFI?

I think the newtype_enum you've added for modifiers is good.