one of the documented procedures in this installation of the ACS
dp_read_checkboxes   { hidden_field_name "_cv" }   { off_value "N" }
What it does:
Reads all checkboxes in the form and sets their value to either Y or N. Note that this function looks for a hidden field named hidden_field_name to get a list of all marked and unmarked checkboxes. This should be used in conjunction with dp_insert_checkbox that sets up the hidden fields automatically.
Defined in: /web/philip/tcl/data-pipeline-defs.tcl

Source code:

    upvar dp_form dp_form

    set size [ns_set size $dp_form]
    set i 0

    while {$i<$size} {
	if { [ns_set key $dp_form $i] == $hidden_field_name} {
	    # list_of_vars will be created if it doesn't exist
	    lappend list_of_vars [ns_set value $dp_form $i]
	incr i

    if { ![info exists list_of_vars] || [empty_string_p $list_of_vars] } {

    foreach cb_var $list_of_vars {
	set val [ns_set get $dp_form $cb_var]
	if {[empty_string_p $val]} {
	    ns_set put $dp_form $cb_var $off_value