The feature has not been implemented in the product yet, however we've released our Sitefinity Thunder
Visual Studio plugin which allows you to easily create designers for existing and new UserControls, and also offers the ability to insert pages, Videos, Images or Document selectors in the designer for fields of type Guid in your control.
All the best,
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items