Since my question above from a few weeks ago went unanswered, I am assuming not a lot of people have attempted this before so for anybody who would possibly care I tried it myself once I was on site. It is unfortunately not possible to walk directly between the two hotels. Behind Renaissance is a wall blocking the way to both Waldorf and Grand Hyatt. So unfortunately mattress runs between hotel chains require to walk from Renaissance hotel to Ploen Chit road and then turn on Radjadamri Road and head to Waldorf, despite both hotels standing next to each other.

There is a small shortcut however, although not as direct as I wished. Instead of walking on the busy sidewalk all the way to Erawan Shrine at the intersection of those two busy roads, there is a small link under Grand Hyatt (near indoor parking) to cut between the two roads. It is easier to walk in the small path there with a suitcase than on the busy sidewalks.