You should probably not be so concerned with chasing the specificity from a trainer. The core and upper body coordination and strength required in a good standing start won't be gained from a fixed trainer. Rather, use the trainer to build your strength and endurance and then once you can get out on track or road, then work on the last points of your start. You can bodge up a start gate type setup or just roll to a stop and then take off. You can train the strength and coordination in a roll-stop, just not the push off