Unusual operator overloading isn't that common in Python, though '%' for string processing is a big and primordial exception. Of the packages I know which use it, only PyParsing uses overloading for something non-analogous to the normal meanings.
I tried hard to think of ways to (ab)use '@', and this XML example and perhaps some sort of messaging API were the only two I could come up with where there was a natural domain-specific mapping.
One of the advantages of keeping '@' at the same precedence level and ordering as '*', '/', '%', and '//' is that it doesn't really offer much in the way of new use. If someone uses '@' for something very unlike matrix multiplication, then they could already use any of the other 4 symbols that way.
So I don't think @ ends up as an attractive nuisance.
