The hot water may pose a problem, although I would leave leftover bloodworms out and they wouldn't turn brown for a couple days. In the worst case scenario, it just doesn't work at all and I just throw it away.
And I was pretty certain I saw something like this in the reptile section, I never heard of that Tetra product, but it seems to be exactly what my idea is. Now that I think about it, there's those "weekend feeders" for fish and I think they're made with gelatin also. I definitely don't see how it could be worse than beef heart and they will hardly be eating any at all considering it'll be a treat since right now they only eat earthworms.
Also, considering its just bones and skin, it wouldn't be much different than what they would eat in the wild, just a different animal of course. I was kinda just making sure that the gelatin doesn't undergo some crazy properties that I don't know about in water and I have an exploded axolotl.