I tried implementing the same customer service xml api that I’ve implemented in sinatra, rails, c# webhandler, nancy etc. I was pleasently surprised how easy it is to get it up and running. There is a story for how you write tests: TestServer, that you install by referencing Microsoft.Owin.Testing.
Previous times that I’ve tried using Owin on mono, there has been some problems getting it to run (I think it was something related to a missing dll). Now it works fine. :)
You can find the code here: CustomerService/Owin
Comments or errors? Fork post and do a pull request on github