I followed this example to display my usercontrol inside Coding4Fun MessagePrompt. http://windowsphonegeek.com/articles/Creating-a-Windows-Phone-7-Trial-Application-Adding-B
I got help from "Eric Fleck - Microsoft" on apphub forums. Here's his solution:
MessagePrompt prompt = new MessagePrompt();
prompt.Body = new WPUC();
prompt.ActionPopUpButtons.Clear();
prompt.Overlay = new SolidColorBrush(Color.FromArgb(155, 41, 41, 41));
prompt.Template = (ControlTemplate)this.Resources["MsgPropmtNoBorder"];
prompt.Show();
Here's the resultant popup:
