There are plenty of projects out there that assist developers in posting things to Twitter; the one I can think of off the top of my head is Twitterizer.
Are there any projects for posting to StatusNet?
Given that StatusNet implements an API very similar to Twitter, I could probably modify/extend Twitterizer to do just that.
However, I'd like to avoid reinventing the wheel if at all possible if something exists already.
You've probably gone through these already, but I thought I'd mention it incase others stumble across it.
As far as I know, there is no specific Status.Net only libraries for .NET, but as you said, the API's are largely very similar because of StatusNet started off as an open-source clone of Twitter.
Last I looked, Linq2Twitter had specific support for Identi.ca/StatusNet, however in MahTweets, we stopped using their library in favour of TweetSharp, which indirectly supports StatusNet via their transparent proxy system.
ie:
FluentTwitter.CreateRequest().Configuration.UseTransparentProxy("http://identi.ca/api/").AuthenticateAs(user, password);
I don't know how reliable either library is going to be at supporting this in the future, as Twitter is set to turn off the basic auth in a month or so. I've not tested if this works for OAuth, since it requires a few more url's to remap.
edit: You can get TweetSharp to work with StatusNet's OAuth api too.
When getting the request token, pass in http://identi.ca/api/oauth/request_token as the TransparentProxy, and pass in http://identi.ca/api/oauth/authorize when authorizing the token. As it currently stands, StatusNet doesn't support OAuth 1.0a, so you don't have the PIN based authentication that Twitter has for desktop clients.