Thanks for the replies TR and Torgos. I actually never noticed that the default flat rate was...only a default flat rate, and that it could be edited per product.

The flat rate example was just a simple example, though, to demonstrate my inability to hook into the shipping quotes and change the items that it was basing the quotes on, but I'd also need this to work with any of the other shipping modules enabled. I don't know how all of the existing shipping quote modules work, but I don't think just setting 0 weight and dimensions would do the trick. I don't think I'll be able to do a (total shipping quote) - (free products shipping quote) type of thing either.

Torgos, on Ryan's suggestion I started this out using the Product Feature API which would allow you to add "Free Product" as a feature and then dictate the cart behavior based on that. It works nicely, but I'm stuck at this shipping quote snag now.