The REST for ASP.NET MVC SDK contains a Word file explaining how you could extend it by adding a custom format:
ASP .NET MVC provides capability to
return HTML. MVC REST adds out of the
box support to return the two most
popular formats for programmatic
access on the web: XML and JSON. In
addition, you can also handle
additional formats. This section shows
how you can add support for a custom
format such as Atom using the provided
extensibility. The process involves
creating a custom Format Handler, and
registering it to handle requests,
responses, or both. The steps specific
to enabling custom formats are
described below:
- Create a custom format handler that may implement either or both of the
interfaces IRequestFormatHandler and
IResponseFormatHandler.
- Register the custom format handler in global.asax in Application_Start
In the included MovieApp
sample you will find an implementation for AtomFormatHandler
which you could use as a base for adding the JSONP functionality. I've also written a JsonpResult which you may take a look at.