This accessor function returns the editor buffer associated with an editor pane, which can be manipulated in the standard ways with the routines in the editor package.
(setq editor-pane
(capi:contain (make-instance 'capi:editor-pane
:text "Hello world")))
(setq buffer
(capi:editor-pane-buffer editor-pane))
(editor:buffer-insert-string buffer
"Some more text...")