Sorry if I am beating a dead horse here, but I was able to talk my local dealership here in Morgantown, WV to turn off JUST the passenger seat belt chime.
It used to drive my dog nuts as it would chime as she came along for rides. We don't know her full history, but all high pitched chimes such as this noise or smoke alarms send her into a fit of tremors. All it took was me taking her with me and showing the mechanic how distraught it made her and they disabled it no questions asked. I did have to sign a waiver saying I approved them disabling a safety feature built into the vehicle...assume this is to cover their own butts!
It's worth a try to ask your local dealer...though I know not all will do this due to the safety aspect.
Regards,
OC