If you use type "list" as your dataAssist search type.
A multiselect list behaves identically to a group of checkboxes with the same name. So if you use type list as your search type you can actuall use a group of checkboxes instead of the list.
You can then create as many copies of the checkbox as you need. You won't want to use the DataAssist Search wizard, which creates your search page. You should be creating your search page by hand and applying the dataassist search server behavior on the results page.