It turns out that someone else just asked that exact question a couple of days ago, in the readBean forum.
Basically, redbean won't store private properties of any class extension.
The solution was then quite simple:
class Model_employee extends RedBean_SimpleModel
{
private $tempPassword;
public function dispense()
{
$this->salt = cypher::getIV(32);
$this->tempPassword = cypher::getIV(8);
$this->password = md5($this->salt . $this->password);
}
}
This won't store the password in the database, nor create the column. Of course, I have to add a getter() if I want to read the password, but the above solves the immediate problem :)