Well my solution wasn't easy, and it took several attempts to make a page work as expected.
Here is mine: pricebook.dataset.asp
There is actually 2 XML datasets reloaded, one is for the manufacturer filter. I created some custom code to handle the filter.
I did have to modify the Web Assist code slightly to show/hide the other drop downs. There are up to 4 drop downs available depending on the categories selected.
It wasn't easy this way, but without Web Assist changing or adding some code I couldn't see any other way of this working.