#region in XAML

OP NOTE: this was the original right answer and in some ways still is a good and for some best answer including me. BUT this answers the original question best I think.

Visual Studio 2015 finally adds support for regions in XAML. You enclose your regional code like this:

<!--#region RegionName-->

All Your XAML In Here

<!--#endregion-->

This will add a collapse icon to the left of starting line, using which you can expand/collapse the region.

Don't know how handy this would practically be since we can already collapse/expand any node in XAML editor. It's there anyway if you want to use.

I wrote a Visual Studio Extension to do this.

I might be missing the point of your request, but the XAML editor automatically puts collapsible regions around the elements. Note the nodes in the left rail of the editor window. Click the "-" and the associated element collapses to a single line, much like a #region.

No, but according to this reply by Microsoft:

This is a great suggestion. Thanks very much. We will consider it for a future release

FYI: This one has been updated to work well with VS 2012 http://visualstudiogallery.msdn.microsoft.com/3c534623-bb05-417f-afc0-c9e26bf0e177

And this VS2012 extension does a nice job of beautifying the XAML so attributes automatically sort and align etc. http://xamlstyler.codeplex.com/documentation