ah, I see... I'd forgotten about the canonical, I've never used before.
That could be difficult to arrange, as all categories are dynamic to what the store owner decides they're selling, and also it's one html/php template used across every category page.
Would it involve lots of hard coded ifs to accomodate the different categories, and then revise later down the road if/when the products have changed?
There is a module that can be bought for Xcart called CDSEO that goes some way to sorting this I believe. Maybe I need to cough up for that...