Dave, the easiest thing is to subclass Form (e.g. xowiki::WikiForm) and define your own Form-class (this is the oo recommended way to extend a class). Then define your own method "new_data" (based on the definitions of WikiForm.new_data and Generic::Form.new_data; the latter.new_data does the "save_new"). There you have full control about the saving operations for new items.
As mentioned before, be aware that you have to care about the validity and security of the passed id, since one cannot trust on from-data.
Jim: i see you point, using the fresh item_id as a "poor men's transaction code". i would not rely on this without further checking. The the storage operations in the db are quite late for double-click prevention, and not particularly well suited for meaningful error messages concerning double-clicks.
