Per the latest answer here, it appears that "menu dependencies" have been abandoned. Is this true?
As an alternative, is it possible, via the client-api to detect when the user has made a selection from one Dynamic Data Source so that I may manually submit the rule, inspect the incoming object and then filter another Dynamic Data Source using the technique described on that same page ?
The next major version of the Code Effects rules engine will include a version of menu dependencies. It'll have some limitations but at least it'll be there. That decision has been made just a month ago due to the high demand for this feature. So the comment you refer to is no longer valid.
As to the current version, every imaginable effort to implement some sort of dependency has failed for various reasons. That includes all versions of the scenario you are describing.