Thanks for the bug report. I agree that this would indeed be a useful feature to have.

This is the solution:

function BL_Config_getParentProjectIds($projectId, $arReturn=array()){
$parent_projectId = project_hierarchy_get_parent($projectId, $arReturn);

if($parent_projectId > 0){
    $arReturn[] = $parent_projectId; 
    $arReturn = BL_Config_getParentProjectIds($parent_projectId, $arReturn); 
}

return $arReturn; 

}

function config_get( $p_option, $p_default = null, $p_user = null, $p_project = null ) {
global $g_cache_config, $g_cache_config_access, $g_cache_db_table_exists, $g_cache_filled;
global $g_cache_config_user, $g_cache_config_project, $g_project_override;

# @@ debug @@ echo "lu o=$p_option ";
# bypass table lookup for certain options
$t_bypass_lookup = !config_can_set_in_database( $p_option );

# @@ debug @@ if ($t_bypass_lookup) { echo "bp=$p_option match=$t_match_pattern <br />"; }

if( !$t_bypass_lookup ) {
    if( $g_project_override !== null && $p_project === null ) {
        $p_project = $g_project_override;
    }
    # @@ debug @@ if ( ! db_is_connected() ) { echo "no db "; }
    # @@ debug @@ echo "lu table=" . ( db_table_exists( $t_config_table ) ? "yes " : "no " );
    if( !$g_cache_db_table_exists ) {
        $t_config_table = db_get_table( 'mantis_config_table' );
        $g_cache_db_table_exists = ( TRUE === db_is_connected() ) && db_table_exists( $t_config_table );
    }

    if( $g_cache_db_table_exists ) {

        # @@ debug @@ echo " lu db $p_option ";
        # @@ debug @@ error_print_stack_trace();
        # prepare the user's list
        $t_users = array();
        if( null === $p_user ) {
            if( !isset( $g_cache_config_user ) ) {
                $t_users[] = auth_is_user_authenticated() ? auth_get_current_user_id() : ALL_USERS;
                if( !in_array( ALL_USERS, $t_users ) ) {
                    $t_users[] = ALL_USERS;
                }
                $g_cache_config_user = $t_users;
            } else {
                $t_users = $g_cache_config_user;
            }
        } else {
            $t_users[] = $p_user;
            if( !in_array( ALL_USERS, $t_users ) ) {
                $t_users[] = ALL_USERS;
            }
        }

        # prepare the projects list
        $t_projects = array();
        if(( null === $p_project ) ) {
            if( !isset( $g_cache_config_project ) ) {
                $t_projects[] = auth_is_user_authenticated() ? helper_get_current_project() : ALL_PROJECTS;
                $t_projects = array_merge($t_projects, BL_Config_getParentProjectIds($t_projects[0])); 
                if( !in_array( ALL_PROJECTS, $t_projects ) ) {
                    $t_projects[] = ALL_PROJECTS;
                }
                $g_cache_config_project = $t_projects;
            } else {
                $t_projects = $g_cache_config_project;
            }
        } else {
            $t_projects[] = $p_project;
            $t_projects = array_merge($t_projects, BL_Config_getParentProjectIds($p_project)); 
            if( !in_array( ALL_PROJECTS, $t_projects ) ) {
                $t_projects[] = ALL_PROJECTS;
            }
        }

        # @@ debug @@ echo 'pr= '; var_dump($t_projects);
        # @@ debug @@ echo 'u= '; var_dump($t_users);

        if( !$g_cache_filled ) {
            $t_config_table = db_get_table( 'mantis_config_table' );
            $query = "SELECT config_id, user_id, project_id, type, value, access_reqd FROM $t_config_table";
            $result = db_query_bound( $query );
            while( false <> ( $row = db_fetch_array( $result ) ) ) {
                $t_config = $row['config_id'];
                $t_user = $row['user_id'];
                $t_project = $row['project_id'];
                $g_cache_config[$t_config][$t_user][$t_project] = $row['type'] . ';' . $row['value'];
                $g_cache_config_access[$t_config][$t_user][$t_project] = $row['access_reqd'];
            }
            $g_cache_filled = true;
        }

        if( isset( $g_cache_config[$p_option] ) ) {
            $t_found = false;
            reset( $t_users );
            while(( list(, $t_user ) = each( $t_users ) ) && !$t_found ) {
                reset( $t_projects );
                while(( list(, $t_project ) = each( $t_projects ) ) && !$t_found ) {
                    if( isset( $g_cache_config[$p_option][$t_user][$t_project] ) ) {
                        $t_value = $g_cache_config[$p_option][$t_user][$t_project];
                        $t_found = true;

                        # @@ debug @@ echo "clu found u=$t_user, p=$t_project, v=$t_value ";
                    }
                }
            }

            if( $t_found ) {
                list( $t_type, $t_raw_value ) = explode( ';', $t_value, 2 );

                switch( $t_type ) {
                    case CONFIG_TYPE_FLOAT:
                        $t_value = (float) $t_raw_value;
                        break;
                    case CONFIG_TYPE_INT:
                        $t_value = (int) $t_raw_value;
                        break;
                    case CONFIG_TYPE_COMPLEX:
                        $t_value = unserialize( $t_raw_value );
                        break;
                    case CONFIG_TYPE_STRING:
                    default:
                        $t_value = config_eval( $t_raw_value );
                }
                return $t_value;
            }
        }
    }
}
return config_get_global( $p_option, $p_default );

}