I think the melting process would happen pretty quickly if you didn't insulate it. , leaving you with the usual amount of lukewarm refreshment. If it was cold enough to deter melting, you don't need the cold liquid. For backpacking, too complicated and extra weight. I've carried a Hydroflask in a cage on the bike and it was a luxury to have ice cold sips for a few hours. Doesn't have a bite valve, though. Gotta stop or be a hands free pro. I've also used it backpacking on the quick over-nighters in our trail infested neighborhood. I make my coffee at dinner time and don't have to fuss in the morning when we're trying to get out quick or hit the trail at first light.
I'll stick with stainless and bombproof. I can very well be wrong as to how long it would stay icy, though. I'd freeze a single bladder if you have it and see how long it stays frozen in your pack.