How do I access BIG-IP "Data Group List" class as a list in an iRule?

You can use any of the TCL list commands (llength, lindex, etc) with a data group. For looping through a class called ClassList, you could do the following:

set count [llength $::ClassList]
for {set i 0} {$i < $count} {incr i} {
  set val [lindex $::ClassList $i]
  log local0. "ClassList($i) : $val"
}

Or you could do it with a foreach loop like this:

foreach val $::ClassList {
  log local0. "ClassList Item : $val"
}

Hope this helps...

-Joe