The PDG Bentley tilts.   If you want whatever is mounted to stay with the user when they tilt, you have to mount to the seat base

 This image shows the bare Bentley seat base

The piece fastened around the footrest support is a Rehadapt  11.1065

One could also use a Rehadapt 11.1075 using the two mounting holes show to the right of 11.1065