You can do this:

al_set_config_value(cfg, "foo", "bar", al_cstr(str));

See al_cstr for a list of warnings. (al_ustr_to_buffer or al_cstr_dup could be used to avoid those problems.)

From an end-user perspective, the only thing that is relevant is that they are saving the file with UTF-8 encoding. Then you'll get the support "for free."

e.g.,

int main()
{
  const char euro[] = {0xe2, 0x82, 0xac, 0};
  ALLEGRO_USTR *str;
  al_init();

  str = al_ustr_new(euro);

  printf("%d\n", al_ustr_length(str));

  return 0;
}

reports 1 as the length of the string, as it should. It doesn't matter if the source comes from a hard coded string or the result of a config file read.