If your goal is to disable the automobile unless the seat belt is buckled, it is hard to envision a detector based on weight or temperature that would be foolproof, as already discussed in this thread.
I would also be extremely pessimistic about any system that would disable the car that the driver could not override. There are just too many hypothetical circumstances, perhaps ones in which a life hangs in the outcome, to not allow an override. At its simplest, all the driver needs to do as an override is buckle the seatbelt and put the object in the seat. Once you allow driver override, then the system is de facto not mandatory.
Nevertheless, a different approach to consider is detection of mass and organic volatiles (i.e., an electronic nose, "voc"). You could put the weight sensor in the seat as usual. The voc detector could be in the seat or in the back rest.
This first article is not related to your project specifically, but I found the history of scent detection quite interesting:
**broken link removed**
This article describes various approaches to electronic scent detection. I suggest reading about SAW and metal oxide detectors specifically. GLC and MS are quite sensitive, but would also be quite expensive.
https://www.electro-tech-online.com/custompdfs/2013/03/09_4536.pdf
Finally, here is a link to a manufacturer of metal oxide detectors. One might consider a back rest sensor that could detect CO2 (not a voc), which is not likely to released from inanimate objects. There are also numerous voc's associated with humans for which metal oxide detectors may already exist.
https://www.electro-tech-online.com/custompdfs/2013/03/APS_MOS_1109.pdf
John
Edit: Here are links to using metal oxide detectors for skatole and ammonia, respectively.
https://www.electro-tech-online.com/custompdfs/2013/03/imtc00.pdf
https://www.electro-tech-online.com/custompdfs/2013/03/detectingand_33.pdf