Answered over on the MSDN forums: http://social.msdn.microsoft.com/Forums/vstudio/en-US/f9e984e3-a47b-42b0-b9bd-1f1c55e4de96/getting-all-running-instances-of-a-specific-class-type-using-reflection?forum=netfxbcl
Not possible :(
Though they at least give you an idea of creating a static List to hold all references of the type (provided its your type of course) that you add to at creation time.