Mixing degrees of publicness in HTTP

At the Data Sharing Workshop the other day, we had a discussion about how to combine OAuth and Feeds, which I was reminded of by Tim Bray's discussion of Adriana and Alec's VRM proposal today.
The session was tersely summarized here, but let me recap the problem.

When you are browsing the web, you often encounter pages that show different things depending on who you are, such as blog, wikis, webmail or even banking sites. They do this by getting you to log in, and then using a client-side cookie to save you the bother of doing that every time. When you want to give a site access to another one's data (for example when letting Flickr check your Google Contacts for friends), you need to give it a URL to look things up at.

The easy case is public data - then the site can just fetch it, or use a service that caches public data from several places, like the Social Graph API. This is like a normal webpage, which is the same for everyone, returning a HTTP 200 response with the data.

The other common case is where the data is private. OAuth is a great way for you to delegate access to a web service for someone else, which is done by returning an HTTP 401 response with a WWW-Authenticate: OAuth header showing that authentication is needed. If the fetching site sends a valid Authorization header, it can have access to the data.

The tricky case is where there is useful data that can be returned to anyone with a 200, but additional information could be supplied to a caller with authentication (think of this like the social network case, where friends get to see your home phone number and address, but strangers just get your hometown). In this case, returning a 401 would be incorrect,as there is useful data there.

What struck me was that in this case, the server could return a 200, but include a WWW-Authenticate: OAuth header to indicate that more information is available if you authenticate correctly. This seems the minimal change that could support this duality, and much easier than requiring and signalling separate authenticated and unauthenticated endpoints through a HTML-level discovery model, or, worse, adding a new response to HTTP. What I'd like to know from people with deeper HTTP experience than me is whether this is viable, and is it likely to be benign for existing clients — will they choke on a 200 with a WWW-Authenticate header?

HTTP does have a 203 response meaning Non-Authoritative Data, but I suspect returning that is more likely to have side effects.