adm_config_set.php has a check that makes sure that a config being added or edited is known before it actually sets it. The config_api is happy to set any config option in the config table. This code doesn't allow managing configs for custom code and configs.

I believe the check was added initially to catch typos. I wonder if it is causing more trouble than benefit.

The fix is to remove the following block of code:

# make sure that configuration option specified is a valid one.
$t_not_found_value = '***CONFIG OPTION NOT FOUND***';
if ( config_get_global( $f_config_option, $t_not_found_value ) === $t_not_found_value ) {
    error_parameters( $f_config_option );
    trigger_error( ERROR_CONFIG_OPT_NOT_FOUND, ERROR );
}