Can't find an API in WebKitGTK allowing me to inject values (not in dom, in webkit) in forms (like most web browser) to enable some input history. As I can't find a WebKitGTK browser doing this, I guess this is missing from API.
Did you try webkit_dom_html_input_element_set_value()?
webkit_dom_html_input_element_set_value() set input value, not input history.