Thanks for the great suggestion. While this would be a useful addition to MVC’s functionality we think an even more performant solution would be to take advantage of the existing IIS static file serving capabilities, which already supports ETags and byte ranges for any static file. Leveraging IIS would mean that the static files can be served from kernel mode, which should be faster than using the managed request pipeline. One way to achieve this is to make sure that the file is in a publically addressable location and then from an MVC action method issue a redirect response that points at the static file.
Since we don't want to have multiple implementations of the same functionality we will not be making this change at this time. However, you might want to consider contributing this code to http://mvccontrib.codeplex.com/.