I gave this a try, but it leaks massively. Like there's no way you can blow up the balloon. To test where the air was coming out, I filled the balloon and then attached it and then put the whole thing under water and watched for bubbles. And there were a LOT! It is the Lego bumps that are doing it.
I was going with 0.2mm and I was sure to have 5 layers top and bottom so that should have been enough. I'm using Cura as my slicer and I'm wondering if there's something about the Lego bumps that throws off the "top" layer setting. It seems like the very top of each bump has a pinhole in it. I'm not sure though... with thousands of bubbles coming out, it is hard to tell exactly where from. I'm going to look at the sliced model very carefully in the morning, but I think I'll just try this again without the Lego feature.