This sounds great. Once you are done with it can you point to an application making use of this form filter? At the moment we always use a different form (ad_form) and pass the filters from the listbuilder (the normal ones) as export variables. Your approach seems to simplify the issue. Additionally, will it be possible to limit the filters used in the form, so you could say -filter_form [list of filters to use]. This would allow you to have the listbuilder in an include and pass the filters to be used on that particular page (similar to row_list).