Sadly not. In general, on_actions can't be triggered directly. Some, e.g. on_death, can be triggered by firing commands that set up the requisite conditions for it to fire (firing the death command on character in this case), but the turncoat blocks have no such easy, time-insensitive trigger. In addition, were one to somehow forcefully trigger the code that handles switching the mercenaries to fight for another character and call the on_action, you would have no way of changing who they'd switch their loyalty to, and if such a calling were to be made possible, it could only ever be used to have mercenaries betray you in a wider variety of circumstances.