Currently we allow casting, by means of a type segment, which implicitly filters or, in expressions in our query options, by using the cast function.
Whereas the cast function allows, optionally, casting to an arbitrary structured type, there is no way to request for an instance of a type and have it, before returning it, be cast to a different arbitrary, typically sibling (read: same base type) type.
BTW if a cast would be not supported/allowed I'd simply expected a 400 Bad Request to be returned by the server.