You shouldn't use the line-lock to stage, it's only used for your burn-out. Roll through the water box, set the line lock and do your burn out. I start in low 2 and let the trans shift, then count to 3 and release the line-lock.
Stage with your normal brake and stage shallow, then set the trans-brake. Mine doesn't move at all, but if it did, I'd still be in stage mode. When it's time to go, just release the trans-brake and you're on your way.
BTW, San Antonio Raceway is IHRA sanctioned and foot-brake means just that in competition. No trans-brakes or electronically switched aides allowed.