Whoa, I am having an 80's flashback! I actually owned a belt buckle knife, it was the type with a heavy brass buckle and the knife fit diagonally across the face. It was a non-locking folder, the pin holding the knife together was basically the bolt holding it to the buckle. It was an interesting novelty but none too secure, plus the blade was small and not much use. If I really needed something for SD swinging the heavy belt buckle would have been the way to go.
Back in the day I saw lots of knives where the buckle was like a shuriken, this is the handle, and the maybe 3" blade fits in the belt, held in by friction. They always looked junky to me, especially because how could you use a knife when you are holding a throwing-star shaped handle? None too comfortable.
Recently I've seen some belt knives that look more serious, I think there's a company called valois or something. Still don't seem all that practical but if a belt buckle knife seems cool to you check them out. Their design is like the 2nd one but the whole buckle (a rectangle of wood or composite I think) is the handle, so you could actually use it.
There's a company making self defense belts that seem to go the route of using the buckle as a flail, probably a bit more practical. Plus, you can probably get that through security.
