I don't full understand your requirements. ASP.NET MVC is also ASP.NET application. Do you have restriction to .NET version? Do you can use only some specific .NET technology?
In general you can mix the main code of your size with the code in other ASP.NET technology. For example WCF service can be the part of ASP.NET MVC or ASP.NET Web Form application. The main code can provide the user interface of your web site and the WFC service will provide JSON data for any Ajax requests from jqGrid.
So you can just use any examples of WCF, ASMX Web Service or even simple ASHX handler.
The demo project from the answer shows how to use ASHX handler in ASP.NET Web Form application.
Another demo project from the answer shows how to use WCF service.
The demo project from the answer or another demo from the answer demonstrate the usage of ASMX web services.
