To get her to fall forward, you have to lift her off the ground, then swing her feet (the same way you dragged her unconscious body, you monster). Make sure she touches back down with her feet behind her.

I wonder if anything in the apartment could be placed in front of her to keep her hips up...

I have not found any thing either. I tried the large boxes and she just falls off as I try to do anything. I have found that if you want to tap that ass while in chains you can. I got her leaning forward at the right height then "jump" in between her legs and have your way. It can be tricky getting back out from her legs and the bar. I had the remote in reachable distance and had to very the height to be able to get her. Also while in between her legs back up to raise her ass then take aim. Attached are screenshots.