Not sure about WP Ecommerce specifically, but here’s an a good tutorial about using a plugin called Cat + Tag Filter. It appears to allow you to accomplish the same thing you’re trying to do.
I don’t use it on my site at http://www.shoppingsignals.com, but I have lots of WordPress Ecommerce experience, and this looks pretty good.
The tutorial:
http://appointzilla.com/create-multi-level-product-filter-navigation-wordpress/
Hope it helps.
Thanks for that tip rbemiller 🙂
It looks like a step towards the right direction however it doesn’t allow the option to choose a custom post type (products) or custom taxonomy (categories for the products). This would also mean no support for WPEC product variations.
I’ll try modifying the plugin and see if I can create something from it.
Has anyone else had any luck with this issue?