That's basically the pattern my company is already using, implementing using our own plugin. We render the full page server-side (using ASP.Net MVC, which is hooked into our server-side Enterprise Application Framework), and then we hook up javascript handlers to manage client-side events. Those handlers can make AJAX requests for content updates, which we deliver either as pure data or server-side rendered html (which are rendered using the same partial templates that the initial page used.)

Depending on the page, we can go anywhere from old-school full-page-refresh to a complete single-page-app design. Mostly we do Enterprise back-office interfaces and customer web portals, both of which tend to have a lot of data grids. So a very common feature of our applications is a data grid that supports paging-in-place, along with server-side filtering and sorting of large datasets. So we'll do that SPA-like, even if the overall site is more of an old-school design where each link loads a new page.

Our plugin does more than just fetch a url; the client side and server side are designed to work together to allow standard options to be passed up to the server along with the request, standardized error handling and content in the response, and standardized ways of handling the response when it gets back client-side.