Figure out a way to support it from above. If the duct is not supported well, if you have to cut a hole in the duct so you can run a wire/ screw/ rod / whatever clear through the duct to a support above it
I hung on and off for 40 years and we never attached to duct work. Vibration from moving air makes a noisy ceiling.
I'd run the mains perpendicular to the duct for best rigidity. I've even installed mains every 2 feet instead of 4 on difficult layouts.
I agree, there is always a way.