Monotouch access private API with return type

Question

I am trying to port the following code to mirror My App's display to an external display via the VGA adapter.

https://github.com/robterrell/TVOutManager/blob/master/TVOutManager.m

This code optionally accesses UIGetScreenImage(); which returns a CGImageRef object. How can i call this Private API from Monotouch? I want to use this method only for an internal build of my app for trade shows.

I have seen several solutions, but none talks about how to call a selector with a return type.