Unfortunately we do not yet support exposing Artificial Fields with WCF Data Services. However, as this is an interesting feature, I have created for you a new feature request in our feedback portal, you can find it here. Feel free to vote for it and for any other features you might be interested in, and it can be included in one of our future releases.
Let me know if you have any other questions or concerns.
the Telerik team
Q3'12 SP1 of OpenAccess ORM packs Multi-Table Entities mapping support. Check it out.