Unfortunately there's no single call that supports this. I've talked to the folks on the Rally support side and I understand that there are performance challenges that prevent them from exposing this via API.
In our mobile application we've worked around this in a couple of ways:
- We provide an alphabetically flattened list of all the projects in the workspace as a path to completeness.
- For hierarchy we query those projects that the user has access to which have no parent and then as the user navigates (expanding the sub-hierarchy) we do subsequent fetches to retrieve the set of child projects for the parent. It's a disconcertingly chatty process but the best that we've been able to come up with thus far