Alright, this is a great point. It would be trivial for me to add addPossibilities. I think I'll do that! Thanks. This will really help for using PourOver to power tagging selection fields where you can ... add possibilities!
Exactly what @rattray said. It's something you get nearly for free in data-binding world and is much harder to accomplish in imperative code. I wish you luck, and make sure you blog about it when it's done! (I want to see how it's implemented.)