I do not know much JavaScript.
But I guess it is possible to use a script that will attach an event for the specified field dynamically( on change value ). I do not really want to create a Zcl_thtnmb*** and write own logic
The event is necessary for the tag <thtmlb:advansedsearch>.
As I understand the form iterator can not be used in this case.
P.S. Sorry for the awful English))