All you have to do is set up a rule to have the screening terms in a certain order. We did on Dawdle in like a day when we realized our in-stock item screening presented the issue: http://www.dawdle.com/search.php/

I was pretty convinced that canonical was a solution in search of a problem, but at least joshu's opening salvo, the DiggBar brouhaha, and the resultant conversation has shown one use for it.