Sounds like the sealing characteristic of milk bathing is the fat content in the milk. I am now curious what half and half cream or heavy cream would yield due to a higher fat content. It was mentioned before what yogurt might bring to the table as well.
Also, I am curious if this process would be a good option for sealing a pit fired pot. Perhaps a vase you would want to have water tight for flowers and the likes. I am thinking a sealed interior, not a surface for eating.