Section 220.127.116.11.5 $root says:
The $root literal can be used in expressions to refer to resources of the same service.
However the ABNF currently limits this to singletons, entities within an entity set, and everything reachable from there, including bound functions:
rootExpr = '$root/' ( entitySetName keyPredicate / singletonEntity ) [ singleNavigationExpr ]
Is this a bug or an intentional omission?