If you set bet persistence to Take SP, then any unmatched bets will be matched at SP at the off.
say you are using a repeating back to lay trigger pre-off:
back matched
lay matched
back matched
lay matched
back matched > goes inplay
lay unmatched > this will be matched at SP
so you do not need a new trigger, just set the lay trigger to Take SP, and the last unmatched lay bet will be matched at SP
the backing trigger's persistence should be set to Cancel
the above is for back to lay