I’ve considered this problem myself previously and was hoping for some insight here. Seeing none I thought I would present my solution. If I was to do this in my own situation, I was going to anchor an attractive piece of wood to the wall, then anchor the towel rack to that. I felt this would pull the towel rack away from the sliding door, reducing the likelihood that it would penetrate the door. The advantage with the pocket door is that depending on the position of the rack to the door, I could actually bolt the wood to the wall rather than using an anchor, since I would be able to reach behind the wall [via the pocket door space, with the door removed] to do it–can’t normally do that!

On our home, after installing the pocket door frame, I then installed the inner wall(on the bathroom side) drywall. Next, pieces of half inch plywood were installed vertically between the vertical pocket door frame members and fastened with construction adhesive to the drywall. The plywood and drywall together were drilled to hold T-nuts with the flange side facing the pocket door. I used T-nuts that had flat flanges with three small nail holes. These were screwed onto the inner plywood face, instead of nailed, using very small screws. Following the above steps, I fastened the towel bars and safety bars from inside the room using chrome plated machine-theaded screws to fit both the hardware and the size of machine threads on the T-nuts. That was nearly 22 years ago, and all continues to function flawlessly.

Back the wall with a 1/2 inch sheet of plywood. I use A/C plywood for its strength. Attach the plywood to the pocket door frame like you wood drywall. Then apply the drywall over the plywood. In areas of the wall were you have regular framing you can rip 1 1/2″ strips of plywood to fir out the normal studs so the drywall can be applied seamlessly.

This method of framing will allow you to mount anything you want any wear you want. If you are installing grab bars for safety please use 3/4″ A/C plywood to back the wall.

This practice works well in for bathroom remodels as in general I find that down the line you don’t know were you might want to hang things and if you have backed your walls you don’t ever have to worry.

This also a good strategy for tub and shower walls. Use marine grade plywood instead. This is great for the handicapped and elderly.
garb bars any wear anytime in the future.

Good luck.

Tom