Current RadGrid (Q3 2015) does supports model binding of master/detail records.

Request the functionality to set ItemType on the detail gridview and pass the master's datakey as the id for the detail select method.

Or, alternatively, if the model has a virtual IQuerable binding, set the detail view's datasource to the models virtual entity

For example, a model with a one to many relationship to the color options for a car:
Model Cars()
{ string name
int colorRange
virtual colorOptionsModel colorOptions
}
Model colorOptionsModel
{
int colorRange
 string colorName;
} 

In the RadGrid:
<mastertable DataKeyName = "colorRange">
<DetailView datasource="Item.colorOptions">