Replace text in XamlPackage

You can use the Razor Engine to do whatever you want in the templating subject. You can download from nuget (http://www.nuget.org/packages/RazorEngine) and without any setup configuration you can use the Razor syntax to do this. For example your template can be this:

<Window x:Class="<class>"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="@Model.Title"
        Icon="@Model.Icon">
  <Grid>    
  </Grid>
</Window>

Note: the @Model.Title and @Model.Icon thats come from Razor

Actually i use RazorEngine to all my templating task:
Email, Report generation on the fly (rdlc), etc…