We just posted a solution to consider for the F.A.Q. of how to enable isolatable WinForms applications.  Check out this link.  This solution may be considered relative to my previous post and subsequent comments.  Note that WPF startup performance costs may be mitigated by applying the [LoaderOptimization(MultiDomainHost)] attribute to the main method.  This tells the loader that the app expects to load multiple AppDomains with similar code and thus should share the assembly images (WPF NGen images) between AppDomains.