I've been working on something similar to what you are asking for. In fact you can probably do it already using a little documented feature of Iron Python. Just Create a new Silverlight web page project and add a folder to it. Then add
an "app.py" file to the folder. The folder will be your .xap name and the app.py file the entry point. To run it, add silverlight.js and an html page with an object tag pointing to <foldername.xap> (similar to what you get in the .web project
of a normal silverliight application) in the root of the project, set the html file as the project start page and hit F5.
You can then set breakpoints and step through your app.py file. One way to set the applications root visual element in app.py could be as follows:
from System.Windows import Application
from System.Windows.Controls import UserControl
rootControl = UserControl()
#use python code to add controls to rootControl
#or use System.Windows.Markup.XamlReader to read in .xaml
Application.Current.RootVisual = RootControl()
When you are ready, you can use Chiron.exe to create the .xap for deployment.
Sorry, I haven't tested this exactly as my requirements are a bit different but something like that should work.
I'm not sure why there isn't a project template for this as its pretty simple and it took me aaaaaages to work it out as it doesn;t seem to be documented properly anywhere... :(