I'd have to go with the two sports bras suggestion. Make 'em good and tight!

duct tape is a bad idea here!!! Great if you are in a beauty pageant

i would personally get a tight sportsbra and a tight breathable microfiber tshirt... something that is going to do both; hold her in but yet have movement... The pecs need to flex, so they can't be taped down, but i understand how her boobs can affect her range.. but dropping too low is killer on the delts anyway, so there has to be a stop and almost bounce point ya know... that is why they make the powerlifting shirts.. can she use one of those with a tight ass sportsbra under..
I always buy a sportsbra smaller than what i wear in a real bra..

ie: if in a bra i am a 36b, i will buy a sportsbra that is a 34a...

Hope that helps a bit
super