I just posted about the buckle spring mod in the same thread max2 linked to: http://www.tacomaworld.com/forum/2nd...disable-4.html
It's an inelegant solution but if your dealership says that the commands aren't available on your truck, it does work.
I'm pleased, Dog in passenger seat: airbag off, no beeping. Dad in passenger seat: airbag on, no beeping. Success!
I'd be interested to hear if someone with a 2013 TRD Sport had luck turning it off with a TechStream or had luck at a dealership - I don't know if my dealership guys were incompetent or if it's a trim-related option.