Can generally be used, but see [1] for limitations.

[1]: https://github.com/3mdeb/efivar/blob/bsd/BSD_PORTING.md

This uses EFI driver which seem to NOT be included into releases?

The are warnings during build like (not sure if needs to be fixed):

    In file included from ioctl.c:25:0:
    /usr/include/sys/efiio.h:46:0: note: this is the location of the previous definition
     #define EFI_VARIABLE_AUTHENTICATED_WRITE_ACCESS   0x00000010
