Found the answer myself.
Richard Dingwall has an excellent post going through various strategies. I particularly like the FilterAttribute solution. I’m not a fan of throwing exceptions around willy nilly, so i’ll see if i can improve on that 🙂
For the global.asax, just add this code as your last route to register:
routes.MapRoute(
"404-PageNotFound",
"{*url}",
new { controller = "StaticContent", action = "PageNotFound" }
);