So I modified the back end server to provide the updated response to the request.
And the calling program then sent a new request that I was not prepared to answer, because we had never seen it before. Apparently, if you claim that you can handle a certain type of functionality, you get a whole new set of queries. Joy!
The new request is documented (fortunately), but the problem is that -- since the number of custom dimensions in our application varies from entity to entity -- I can't actually send back the "correct" response that would include the set of custom dimensions that we intended to support.
Now, it happens that in order to implement a sparse data set, the custom dimensions get bound to the account dimension, so I suppose I can just report out the five static dimensions that we use and let the custom dimensions bind to the accounts.
I'll think about this some more in the morning.