When debugging a program in Visual Studio 2013 by using an external debugger plugin (such as VisualGDB) instead of the built in debugger, .natvis files are not loaded and used.
Is it at all possible right now to use NatVis with any other debugger than the built in debugger, and if so how does one enable this?
I do not believe what you want is possible. From what I can tell from having used VS's debugger and a couple external, integrated debuggers for various game consoles, it is the debugger's responsibility to collect the expressions for the scoped variables, and to display the proper values for them.
Your best bet would be to either use VS's debugger directly (if you can), or see if you can't get the developers of the debugger you're using to add their own support for type visualization.
Sorry!