Two ideas --
First, why not sew a button on to stop the buckle part sliding down, a sewing needle will go through the webbing without weakening it , and if you use a 2 hole button even less chance of damage.
If you don't want to do that , how about one of those shoulder pads you get to wrap around a seat belt so it doesn't press on your shoulder, you could attach it on the lower part of the belt to stop the buckle sliding down, and no chance of damaging the webbing.