Custom Proxy Factory with WinRT in v.4.5.501

I'm following the Using CSLA Data Portal e-book where it talks about implementing a custom proxy factory so that an app can support occasionally connected scenarios.

The SL example project shows the custom class that implements Csla.DataPortalClient.ProxyFactory. This doesn't appear to be valid interface within the Csla namespace for WinRT in v4.5.501. I do see IDataPortalProxyFactory, but this interface doesn't contain an overridable GetProxy method, which is necessary.

Are there changes in 4.5.501 that changes how I should create a custom proxy factory? Thanks.