I just say one of my competitor's CC model. It was nicely trimmed according to an irregular shape of the site.
Does anyone know how to do it?
While this was not possible at the time you asked it, the very recently released ContextCapture Update 9 allows importing a KML or DGN clip shape to apply to a specific production, and the resulting models are neatly trimmed according to the submitted shape. Hopefully it will make the current process a little smoother for that use case!
What would be the simplest way to create a KML or DGN clip shape? In CC editor using drawing tools?
Yes, from CC Editor you can draw a shape using Place Smartline or place shape, just make sure you close it on itself.