You can set include_path with directory to the public models as default and create abstract controller for all admin controllers with additional path to admin models if needed.
PHP Code:
abstract class My_Controller_AdminModule extends Zend_Controller_Action
{
public function init() {
set_include_path( get_include_path() . PATH_SEPARATOR .
realpath('../admin/models/'));
}
I use it once and it works pretty fine for a small project. You can also put into this controller all stuff used only by admin controllers in order to remove some code from bootstrap.