I used this little snippet after a couple of hours hunting MSDN forums
StreamResourceInfo f = Application.GetRemoteStream(new Uri("pack://siteoforigin:,,,/"+fileSolutionPath));
//f.Stream //to your IO with this
Note: siteoforigin files are downloaded from the host on demand, so should be downloaded as needed.
Cool little beasty
I actually used this to copy some configuration files, DICOM Server Association files, to my Full Trusted XMAP home. I had made the Full Trust XMAP home (Environment.CurrentDirectory) a Temp folder i.e. System.IO.Path.Combine(System.IO.Path.GetTempPath(), "MyAppName") and needed to get the association list for a Storage SCP.