After having a look over the HTTP status codes, 202 still seems appropriate to me. The chunk has been accepted and it will be processed later, definitely not during the current request, and no resource has been created. The only part that is processed synchronously with its request is the final part, which should return 201.
Perhaps the more generic 200 would be more appropriate here if it is not a close enough fit for a 202. We'll have to make sure the client will not be affected by such a change (I don't expect it would be).