Aye, and for custom sites, having a cart product edit form actually shouldn't be too difficult. In fact, maybe it wouldn't be too difficult in core either if we we were able to implement something like this in the future...

1. Edit link takes you to a page that rebuilds the add to cart form.
2. If user hits cancel, no action is taken.
3. If user submits but doesn't change anything, no action is taken.
4. If user makes changes, then totally remove the old item from the cart and let the add to cart form work from scratch. This involves a custom submit handler that has to fire off before the default one to make sure the old cart item is removed. Letting it add itself back like normal will then allow all the other modules hooking into this process to fire off.

If you get a proof of concept working that satisfies the above requirements, I can see this working out great in core. Then the question would be where/how to include the edit link.

A module like this can be created totally in a contributed module using the appropriate hooks, most specifically hook_form_alter(). This will make it public even if we aren't able to include it in core, and it will keep you from having to hack core to get it done (and thereby breaking your upgrade path).