This would be a very useful, and presumably very tricky seal to make. A failure mode for this seal is that you overfill the artery, and your artery explodes. Not fun. In order to mitigate that, I would suggest making it repeatedly release extremely small amounts of blood, rather than trying to shove it all in there at once. Even better, a slow trickle, but I imagine that's even harder to do.
Most importantly, we need a way to test it which doesn't involve a human test subject. It may not matter that your veins are exploding when you're dying anyway, but test subjects who aren't dying would probably frown upon that. I would suggest using an animal model (whatever has a similar blood pressure to a human. I assume that varies based on size or something?). Maybe a large pig or something.
As for your previous idea of sealing away water once it reaches the stomach, one obvious failure mode is if it seals too much water, causing the subject to desiccate, or concentrating the stomach acid too much and it burning its way through the stomach lining. Again, definitely use an animal model for testing. Furthermore, the seal should gauge the level of water in the stomach, and only store more if it gets too high. It needs to only be able to store water, and to not be able to store water that still has chakra in it (so that Cthulhu doesn't come out of Hazou's small intestine).
I would also suggest that it jettison its contents into the Out when it is destroyed. Otherwise when it gets destroyed it may jettison its contents into the stomach, causing the subject to explode.
Overall, I would consider these seals to be very useful, but also very difficult to research.
