Luckily, I stumbled upon a very useful extension for Visual Studio that does exactly this. It is presented as tool for organizing XAML files, but actually it deals with every xml document. I already tried it and it works great with both VS 2010 and VS 2012.
The syntax is pretty much the same as we know it:
<!-- #Region Some Useful Label here --> //your xml document <!-- #EndRegion -->
Check it out: http://inchoatethoughts.com/xamlregions or grab it from Visual Studio gallery. Big Thanks to the author Jacob Johnston!