Windows Cloud Files API provides IStorageProviderUriSource interface (desktop4:ContentUriSource element in manifest). It is implemented in both CloudMirror sample from Microsoft and in Virtual Drive sample from IT Hit.
After running the sample code, I have found that IStorageProviderUriSource methods are called when MS Office application is activated, for example when MS Word is brought to the foreground.
However, the purpose of this interface is still unclear to me. How content ID, that is mentioned in the ContentUriSource documentation can be used?