Hey,
I'm looking into the exact same things these days with my T-Sport.
According to Toyota, if you disconnect one airbag it will shut down the entire system. Not a good idea. If you unplug one airbag the light in the dash will flash, but nothing else happen when you fire up the car. I just did that the other day while I had my passenger seat out. I did not, however, try to put the seat belt in, but I can't for the life of me imagine that anything would happen. As far as the car is concerned there is nobody in the seat anyways.
TRD has airbag warning canceller plug thingy, but this is not listed as being compatible with the T-Sport/Sportivo. But..I have a couple of them TRD plugs on the way in the mail, and I think they will arrive tuesday or wednesday. Then it will take me about 2 minutes to find out if those plugs will do the trick or not...
Alternatively you can, as mentioned, measure the impedance in the seat in order to make your own canceller, but you do run a risk of blowing the airbag. At least that is what everybody has warned me about...
EDIT: I ordered the cancelers from Greenline Motor