That definitely doesn't sound like it's worth it. I found our last event I had the frozen rind with ice cold fruit in it, and the rind started to get soggy after about 2 hours. I was thinking that if I were to instead, deep freeze it in my -20F freezer, it may last longer. That said, the majority of the rind's internal makings is water, so it doesn't sound like it would make much of a difference.
What about things like freezer packs? I've not been able to find any resources as to what's inside them, but I have had one leak once, and it leaked a blue gel like substance. https://www.webstaurantstore.com/cambro-cp1220-full-size-camchiller-for-cambro-food-pan-carriers-camkiosk-and-camcruiser-carts/214CP1220.html
Would something like this benefit more from a -20F since it's not purely water, but some sort of coolant, or am I likely to have similar results as the water rind? So far I've noticed that they work better in my -20f freezer, but to be fair, when I have had to store them in our commercial freezer, it was because I had forgotten to freeze them for 3 days ahead of time in the -20f, so they were typically room temperature when they were put in the 0F freezer, and they were likely only in there for 16 hours or less. The only thing we've tested on these is I had some chicken that had been frozen and defrosted too many times, to where even though it was raw, it would effect the flavor some, so we decided we were going to toss it. We left it in one of our cambro's with this ice pack, and 48 hours later, the chicken still had an internal temp of 37F. These things are quite useful in the food industry!
One other question. You were referring to how long it would take ice to melt, and that -20F would only give it a few minutes longer to get up to 0F. But what about a large block of ice (like what an Ice Sculptor would use)? If you have a large, say 1ft thick block of ice that's all -20F, wouldn't the internal temperature of the core, help to slow the process of the outside from reaching 0f and beginning it's transition into water?