Goofus, I don’t think a negative is a minhag (certainly not in this case).
ZD, firstly, the jacket is more important than the hat; secondly, I understand that some don’t own a hat, and I wouldn’t expect someone to buy a hat for an occasional visit (although I know someone who did). I’m talking about regulars who do own a hat (and I’m not talking about anyone in particular, just the idea). Even if someone was a regular or I knew that they had a hat but chose not to wear it, I wouldn’t say anything either, except hello.