I implemented that. Let's say you want to make a reference qualifier to the for Configuration item on the template. Then you have to create the Before Query Business Rule that looks like this (on cmdb_ci table)
(function executeRule(current, previous /*null when async*/) {
var targetName = gs.action.getGlideURI().getMap().get('sysparm_target');
if (targetName.indexOf('change_request.cmdb_ci') > -1) {
current.addQuery('install_status', 1); // installed
}
})(current, previous);
When you click the magnifying glass, there is a parameter in URL sysparm_target that always differ a little bit. On the change form it's just change_request.cmdb_ci, and on change proposal, it has format like change_request.cmdb_ci.{sys_id format}. So you can detect it like this. Then you are able to provide only specific CIs after clicking on magnifying glass.
You should also take care about the putting the Name of CI directly to the field. You cannot find the CI in the list after clicking on magnifying glass, but when you put the name into the field, it behaves as valid. Here I suggest to use client script with AJAX to check if the user put there valid CI.
