Site-Wide Taxonomies

I need to richly tag all posts and very dynamically filter them based on these tags site-wide.

Let's take a car-site as an example. I need to get all posts related to volvo four wheel drive:

?brand=volvo&feature=four_wheel_drive

I know how to accomplish this for a single site, using something like this: http://wordpress.org/extend/plugins/query-multiple-taxonomies/

How can I query site-wide posts using their taxonomies? Or some other approach?

All input is welcome.