Can I specify a custom location to "search for views" in ASP.NET MVC?

You can easily extend the WebFormViewEngine to specify all the locations you want to look in:

public class CustomViewEngine : WebFormViewEngine
{
    public CustomViewEngine()
    {
        var viewLocations =  new[] {  
            "~/Views/{1}/{0}.aspx",  
            "~/Views/{1}/{0}.ascx",  
            "~/Views/Shared/{0}.aspx",  
            "~/Views/Shared/{0}.ascx",  
            "~/AnotherPath/Views/{0}.ascx"
            // etc
        };

        this.PartialViewLocationFormats = viewLocations;
        this.ViewLocationFormats = viewLocations;
    }
}

Make sure you remember to register the view engine by modifying the Application_Start method in your Global.asax.cs

protected void Application_Start()
{
    ViewEngines.Engines.Clear();
    ViewEngines.Engines.Add(new CustomViewEngine());
}

There's actually a lot easier method than hardcoding the paths into your constructor. Below is an example of extending the Razor engine to add new paths. One thing I'm not entirely sure about is whether the paths you add here will be cached:

public class ExtendedRazorViewEngine : RazorViewEngine
{
    public void AddViewLocationFormat(string paths)
    {
        List<string> existingPaths = new List<string>(ViewLocationFormats);
        existingPaths.Add(paths);

        ViewLocationFormats = existingPaths.ToArray();
    }

    public void AddPartialViewLocationFormat(string paths)
    {
        List<string> existingPaths = new List<string>(PartialViewLocationFormats);
        existingPaths.Add(paths);

        PartialViewLocationFormats = existingPaths.ToArray();
    }
}

And your Global.asax.cs

protected void Application_Start()
{
    ViewEngines.Engines.Clear();

    ExtendedRazorViewEngine engine = new ExtendedRazorViewEngine();
    engine.AddViewLocationFormat("~/MyThemes/{1}/{0}.cshtml");
    engine.AddViewLocationFormat("~/MyThemes/{1}/{0}.vbhtml");

    // Add a shared location too, as the lines above are controller specific
    engine.AddPartialViewLocationFormat("~/MyThemes/{0}.cshtml");
    engine.AddPartialViewLocationFormat("~/MyThemes/{0}.vbhtml");

    ViewEngines.Engines.Add(engine);

    AreaRegistration.RegisterAllAreas();
    RegisterRoutes(RouteTable.Routes);
}

One thing to note: your custom location will need the ViewStart.cshtml file in its root.

Now in MVC 6 you can implement IViewLocationExpander interface without messing around with view engines:

public class MyViewLocationExpander : IViewLocationExpander
{
    public void PopulateValues(ViewLocationExpanderContext context) {}

    public IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context, IEnumerable<string> viewLocations)
    {
        return new[]
        {
            "/AnotherPath/Views/{1}/{0}.cshtml",
            "/AnotherPath/Views/Shared/{0}.cshtml"
        }; // add `.Union(viewLocations)` to add default locations
    }
}

where {0} is target view name, {1} - controller name and {2} - area name.

You can return your own list of locations, merge it with default viewLocations (.Union(viewLocations)) or just change them (viewLocations.Select(path => "/AnotherPath" + path)).

To register your custom view location expander in MVC, add next lines to ConfigureServices method in Startup.cs file:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<RazorViewEngineOptions>(options =>
    {
        options.ViewLocationExpanders.Add(new MyViewLocationExpander());
    });
}