Once I was able to Use Jni4Net (Part 1) - To C# REPL a java process (ZAP Proxy) I was able to use the ZAP BeanShell support to start controlling and manipulating ZAP's GUI from the .Net C# REPL.
Here is an example of adding a new 'Site' (i.e. a TreeNode) to the main ZAP 'Site's window (i.e. TreeView)
For more details on how this PoC was created, take a look at this pdf: