An old post I know, but I happened to be playing around in this area, and have successfully run a WebMatrix razor ASP.Net Weg Page within a mojoPortal site. To include this onto a true mojoPortal page I had to use an iframe feature, with the URL set to the cshtml page.
My cshtml page pushes out a data grid from an OData service, so I dropped Microsoft.Web.Helpers.dll and Microsoft.Web.Helpers.OData.dll into the \bin folder of the site, with no apparent ill-effects. The dlls for Web Pages and Razor were already present.
I know this is not an ideal architecture but it may answer the OP.