Setting the Content property of DataFormDataField doesn't take any effect. The value should be displayed in the editor are of the data field.
To work this around, you can use a custom data field.
public class CustomDataFormDataField : DataFormDataField
{
protected override Control GetControl()
{
if (this.Content != null)
{
return this.Content as Control;
}
return base.GetControl();
}
}