No, it should not be legal. To prevent removing sport BJJ further from a real-life applicable form of combat.
Fighting in gis are fine by me. Everybody on the str33t wears clothes of some form you can grip, even more so in cold climates like where I live.
However, the point of the belt is to keep your gi together and to denote your rank. It is not designed for attacking. When was the last time you saw a streetfight in which belts had any importance at all? I have never seen one, whereas I have seen plenty where clothes played a part, validating training in gis.
Well, if you wanna get technical, only very sturdy jackets could come close to a gi top in JJ. Jeans could be comparable to gi bottoms, but you would really only use that for a toriena guard pass, or in a similar limited situation. On the other hand, what percentage of the population does not wear a belt. A belt buckle is also not a terrible improvised weapon either.
A belt that long? A belt that easy to remove? A belt that durable?
And the topic is not guard passes, the topic is belt choking, which should NOT be allowed in competition in any way if you ask me, because it just will not happen in any fight you’d be able to get into, unless you’re some kind of ninja.
Nor the belt itself. Nice thick leather strap. With a little practice you could get a pretty good whipping motion out of it, like snapping a wet towel, only heavier and leather. Depending on the type of buckle it could be slow on the draw though.