Hi
As long as all selections back prices are less than 1000
In the price field: IF(back_price>25, 1000, back_price)
If the current back price is greater than 25 this will place an unmatched back bet at a price of 1000 which will be matched at BSP at the off. If the current back price is 25 or below it will operate as per your trigger at the moment. If per chance the 1000 bet gets matched before the off you could be a lucky man.
As this trigger is relying on SP it may be worthwhile putting in a condition to check the market supports SP:
and markets SP is supported
The other way to do this is two triggers, one for back prices less than 25 just like your current trigger, with another condition:
and selections back price is equal or less than 25
and another trigger to back at SP
condition: selections back price is greater than 25