Hi
You should be able to do that but depends how you want to take SP.
As you are now, with persistence for the back bet set to Match at SP, if there is any back_unmatched at the off, it will be matched at SP
your attempt to greenup trigger should then execute only in-play, with perhaps this condition added:
selections back unmatched is equal to 0
Or you could try a "back at SP" backing trigger with optional price limit set
attempt to greenup should execute in-play
Please use test mode when testing