I released version 0.8.0 of XML-RPC.NET last night. It contains support for optional struct members but the FAQ documentation is not updated in this release. However this blog entry shows how optional members are used in practice.
Note that new nullable types such as XmlRpcInt can be used where a struct returns optional value types such as integer. Otherwise you would not be able to determine whether the member had not been set or was set to its default value. Similarly when passing a struct in a call, the nullable types enable you to specify whether a member should be included in the XML-RPC struct in the request.
The release also includes some interfaces for the following APIs:
- Movable Type extensions
The interfaces are work in progress but I've been using all of them except the Movable Type extensions in clients.