I think the idea is with that kind of trigger, the DataAssist results page can be contained to one page and based on whether that trigger is present or not, can then be filtered or not filtered.
But instead of changing your form submit button, why not just add a hidden field with the needed value for DataAssist search to be triggered? If my mind serves me correctly, that's what happens on the automatically generated search by DataAssist.
Best regards,
Brian