This sort of fell out of the sky and hit me. Five years late. Maybe you've found or figured this out long ago; maybe you've struggled with the problem and have been searching for a solution. So, for what it's worth.
Since taking purchase of my 2018 SR5 w/3rd row and a cargo area privacy cover, I've hated the rear seatbelts. They pose a daily nuisance and struggle to open and close the privacy cover. The tangs of the cover snag the belts, hanging up on them, often pulling the belts out of the clips that hold them to the walls. The privacy cover plastic handle has taken the brunt of this, now unusable as it has comes loose and it pulls out from the cover if used (it's still in place but easily pulls off). The belts also work themselves out of the clips on a regular basis during daily driving.
So this morning, dropping the 2nd row and clearing the cargo area to go to the ranch store for some bulky items, I found one of the belts had somehow snagged the back of the fender well housing and had caught up on it. I went to put it back then realized that there was considerable friction and snugness keeping it in place. So I duplicated the position of the belt on the other side.
They appear to hold well, so I'll try it. This position won't really help with snagging the privacy cover, but at least I shouldn't have to put the belts back in their clips regularly after snagging. The belts are held pretty snug in the clip on the wall.
This is a trial run. If this doesn't work in the long run, I'll update this. The real solution is to remove the belts, but the 3rd row has been used infrequently.