While I was thinking about the issue last night, that's almost the exact solution I came up with. I'll do one per nid, just to seed the database table. Serialized arrays and all that.

I'll be finishing it up today and posting as a contrib, with any luck.

Ryan, while I have your attention: is a theme_override function the way to go for this? Or is there a better way we can hook into that page? Not that copying and pasting the view_cart function whenever there is an update is THAT hard - but I feel like there has to be a way to do this right from the Contrib in a universal way, so that it doesn't have to be done on a per-theme basis. Maybe I'm missing something basic.

Perhaps a hook_uc_view_cart_form that will get called at the end of viewing the form contents? I don't really see any other use for such a thing, though... maybe my Contrib will get considered for Core inclusion one of these days (once it's done)