I spoke to @dmitchell and he explained your concern to me that getting the parent is a more expensive operation than I'd realized. In split mongo the data structure is all in memory and so there should be no cost. For now, this breadcrumb is only going one level up, so this overhead shouldn't be too bad. I prefer to keep a cleaner API where I can deal with the xblock directly rather than passing locators/locations around and having to understand them.

Let me know if that doesn't seem reasonable.