Currently, the Working Group has no plans to extend the federation extension to cover SPARQL update. There are several challenges with federated update that lead the group to conclude that the area is not yet mature enough for standardization.
The main problem of using the federation extension with update (and other extensions) is that you always rely in the remote server capabilities like security issues. Such security issues may involve security certificates or HTTP access and authorization. If HTTP access is used, credentials have to be sent and the SERVICE operator currently does not support that.
Another issue is the purpose of updating multiple updated endpoints. Then the operation may as well be executed as a set of separate operations. SPARQL has no concept of transactions, meaning that there can't be any guarantee of integrity across endpoints no matter what we do. It could be argued that the WHERE clause could be resolved into a single image that is used for consistent updates across stores, which may make integrity more likely, but it still can't be guaranteed.
We would be grateful if you would acknowledge that your comment has been answered by sending a reply to this mailing list.
Carlos, on behalf of the SPARQL-WG