With a 500E rear buckets you will need a hole for bolt attachment located on either side of the car center line. If I look inside the trunk and under the rear shelf area, my w124 body has several small holes viewable from inside the trunk that are near the rear seat headrest area, midway between the rear speaker and first aid kit. I can see the bottom of the felt-like fabric wehich covers the shelf through these holes. I imagine one of these could be enlarged to accept a child seat anchor bolt. But I share your skepticism about how much additional safety this strap will afford in most real life accidents, short of a complete car rollover.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E