I'd intended to post this as a question, but I've now got a solution that seems to work fine. About two years ago, the seat belt tongue stop broke off. (This is the little plastic button that prevents the metal tongue that fits into the buckle from sliding to the floor when the belt is retracted.) I'd tried sewing a button on to replace it, but the button broke and I was back where I started. So today I went searching for this silly little part and, through the wonders of Google, finally found out what to call the thing.
I'd been worried about poking too big a hole through the fabric, but once I saw the thing I knew that it would make a hole about 1/8" diameter. So I decided that a pop rivet could be used instead. Did. Works.
Others have suggested a diaper safety pin, which should also work.
If you have to buy the real thing, you now know what to call it and can go to a dealership and spend a whole buck to have your very own.
Hope this helps some frustrated fellow traveler!