I know I've seen crochet snugs for tortoise that fit around the tort and have a pom pom on top, usually brightly colored. Never balloons though! My torts would probably throw a fit lol
I don't think it's cruel at all. When I first got my Leo, I was trying to think of how I could put one of those tall bright orange flags that you used to see on bike, on him. Lots of panic attacks trying to find where he was hiding. I think one of those kids GPS would be good too. Double tape it to the tort My torts bigger now, so I can find him easier.
I have used a long ribbon taped to them before. So long as its not damaging the animal with no risk od lifting it up up and away then I think its great idea.