My tentative plan, which I've not gotten very far through, was this:

1. Add a product feature "Free Product"
1a. Add conditions for this to be a free product (another product is in cart, order total is over a certain amount, B1G1)
2. In the cart, display product as normal but if conditions are met, set price to $0
3. In checkout, if only free products are in the cart, streamline the checkout
4. In checkout, if priced and free products exist, free products will have a price of $0 and won't affect shipping or taxes, but checkout will proceed as normal otherwise.

I think I can do everything except #4. I'll have to take a look at how discount and gift certificate mods do $0.00 total order processing too and probably tie some of that in with #3.