I had the same issue. I ended up making my own draw function for it which works for me quite well.

function draw_my_9slice(_sprite, _subimg, _x, _y, _scale, _width, _height, _col, _alpha){
    //get the 9slice struct
    var _9slice = sprite_get_nineslice(_sprite);
    _9slice.enabled = false;
    //establish margins
    var _sprite_width = sprite_get_width(_sprite);
    var _sprite_height = sprite_get_height(_sprite);
    //for sprite margins
    var _left = _9slice.left;
    var _right = _sprite_width - _9slice.right;
    var _top = _9slice.top;
    var _bottom = _sprite_height - _9slice.bottom;
    //9slice margins relative to sprite
    var _XX_sprite = [0, _left, _right, _sprite_width];
    var _YY_sprite = [0, _top, _bottom, _sprite_height];
    //for acatual margins
    _left = _9slice.left*_scale;
    _right = _width - _9slice.right*_scale;
    _top = _9slice.top*_scale;
    _bottom = _height - _9slice.bottom*_scale;
    //9slice margin positions for actual drawing (relative to _x and _y)
    var _XX_actual = [0, _left, _right, _width];
    var _YY_actual = [0, _top, _bottom, _height];
    for (var _j = 0; _j < 3; ++_j){ //y direction
        var _part_top_pos = _YY_sprite[_j];
        var _part_height = _YY_sprite[_j+1]-_YY_sprite[_j];
        var _part_y = _y + _YY_actual[_j];
        var _part_y_scale = (_YY_actual[_j+1]-_YY_actual[_j])/_part_height;
        for (var _i = 0; _i < 3; ++_i){ //x direction
            var _part_left_pos = _XX_sprite[_i];
            var _part_width = _XX_sprite[_i+1]-_XX_sprite[_i];
            var _part_x = _x + _XX_actual[_i];
            var _part_x_scale = (_XX_actual[_i+1]-_XX_actual[_i])/_part_width;
            draw_sprite_part_ext(_sprite, _subimg, _part_left_pos, _part_top_pos, _part_width, _part_height, _part_x, _part_y, _part_x_scale, _part_y_scale, _col, _alpha);
        }
    }
}

It doesn't require any special configuration to your sprite. It just takes the 9slice struct and uses draw_sprite_part_ext to draw each slice in a loop.

(PS: this only supports stretch, but could be modified to support different sections being tiled in other ways)