I wanted to achieve the same thing on a site I built last year, but couldn't find any way to do it. In the end I hacked it a bit as follows:
1. Get the shop to list a load of items on a page (using tags to group them).
2. Click on an 'add ALL to cart' button which, in turn, fired off all the AJAX 'add to cart' buttons for each product in the list.
3. At the same time, it started a timer which waited long enough to make sure all the items had added to cart by AJAX.
Will try out your group suggestions and get back here if I can make it work.