I'm not sure if i see much benefit over just having a header on the 200 response. I suppose if it takes a bunch of time to generate the response body, but most of the time where this sort of thing would make a difference you probably already have the response body for the main request cached on the server side.
I guess the advantage is that you can send this before you know the status code of the response? For example if you know what CSS+JS file the client will need but still need to generate the page which may 500, 404 or 302.
