Saw a good flier three years ago using baby powder as you described. His setup was with a sliding door, rather than a swinging bomb-bay door arrangement. This allowed him to open/close the door proportionally to regulate the amount of powder dropped.
I said he was good, cause he'd center-line the runway at 10' or less spraying the powder, cut the powder off, perform a stall turn and repeat until he was out of powder. Very impressive :spinnyeye