Using Seat Belt Cushion/Sleeve/Pad on Seat Belt?
 

Does anyone please know if it's ok to use a seat belt cushion/pad/sleeve on a standard airplane lap belt (something like this)?

I assume it is, because many keep their seat belt loosely fastened, or fasten them over blankets, etc., but I'm also aware I've never seen anyone use one, nor can I find any useful information online about whether or not it's allowed. It's for a medical reason, and if it's a case of contacting the airline then I will, but I was just wondering if anyone on the forums has used one on a flight, and/or knew whether it was ok/broadly accepted to do so.

Thanks.