Not at the moment, though I've had it on my mind for awhile and I filed an issue to address it. Feel free to submit ideas about the API there.
I'm not sure if the cache should be on the runtime or the document. If it's on the runtime, we need to make sure it's protected by the the thread_safe collections.
Currently, you can use the references property (of type Hash) on the document object, though it's not specifically designed to be a generic cache (it could get cleared in certain cases).
-Dan