It is not an uncommon practice to use an interface as a container for public static methods.
However, adding @Function to a public static method in an interface results in a compilation failure:

[ERROR] Somehow this method got enclosed by something other than a class

This is a rather unhelpful message to being with: Which method are you talking about? Who is telling me this?

But as far as I can tell, PL/Java has no need to instantiate such an outer class at all, so there seems to be no real reason why it can't be an interface; that should not affect the ability to invoke a contained static method.