I am still not sure where GtkField comes from. I think you mean GtkEntry. Anyway, I think I know what you are asking.
I don't think there is a method to swap out the search widget with one of your own. Christian's dev_inspector may help you find a way to pull it out of the treeview's (grand-)children.
This is how I would do it:
First, turn off interactive search. Then check for changes in your GtkEntry by connecting to one of the GtkEntry signals (probably insert-at-cursor) and then search the model (GtkTreeModel::foreach()) from your GtkTreeView. When you find a value you can scroll to the cell your are looking for (GtkTreeView::scroll_to_cell()).
Oupss, yes it was GtkEntry (I'm a bit tired :s).
It seems to be a solution to my problem, thanks for your answers !
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Marion DESNAULT