Note that an alternative could be using an existing structure from an ACPI-CA header and populating the fields using htoleXX() when needed, e.g. something like:

ACPI_TABLE_FACS facs;

memset(&facs, 0, sizeof(facs));
/* Populate fields */

BASL_EXEC(basl_table_append_bytes(facts_table, facs, sizeof(facs));

Note that actbl.h explicitly notes that the FACS does not use the common ACPI table header (ACPI_TABLE_HEADER).