Suppose another spec wanted to define when @RequestScoped applied to its
operations, how would it do that? The javadocs for @RequestScoped seem to
be an exhaustive list, not allowing the scope to be used in other contexts.

The javadocs need to indicate that the scope can be active at other
times beyond what the spec describes. It can be tricky to do that in a
way that doesn't allow people to implement the currently defined scopes
incorrectly.