Finding Relationships via Web Service

I’m working with the web service to load/update content items. In some cases, I also need to delete them.

Before deleting a content item, I first need to verify that no other content items in the system have any references to it. Reading through the web service documentation, I find two promising methods in the System Relationship Services: FindRelationshipDependents and FindRelationshipOwners.

Problem is, neither of these methods appears in the WSDL. I did find a very similar FindDependents and FindOwners, but the former returns nothing (even though another item definitely refers to my test item) and the latter only returns the folder my test item belongs to.

Is there another set of methods which might be more appropriate?