Try hooks.

Hooks on the pocket door wall or under the window.

If you can still access the back of the drywall where the pocket door will go, you can perhaps secure a plywood sheet, or a piece of wooden blocking, between the studs to give you something to screw into. This will depend on the wall thickness and the pocket door design/width. Alternately, how about a floor mounted, heated towel rack?

I have an over the door type hook hanging on my glass shower.

I have a towel bar on my pocket door wall. It screws into a 3/4" piece of plywood that is attached to the back of the sheetrock with construction adhesive. I got access to the back of the sheetrock by inserting the plywood from the opposite side of the wall before it, too, was sheetrocked over.

Another option is to locate the wooden slats that form the frame for the pocket door. Just use screws that don't exceed the combined thickness of the sheetrock plus the wood. It will be plenty secure. The only limitation is that it will restrict the location and length of the towel rack. That's why blocking gives more flexibility.

I like the "train racks". This is just one example:

https://www.potterybarn.com/products/mercer-train-rack/?catalogId=84&sku=2978760&cm_ven=PLA&cm_cat=Google&cm_pla=Bath%20%3E%20Bathroom%20Hardware&cm_ite=2978760&gclid=Cj0KCQjw2K3rBRDiARIsAOFSW_4PWSI3wJyf5yGhL91vwmFKVzfoLM9aMsheCKCBrBxCac7_t1wc2v8aAmKDEALw_wcB

There are towel racks with brackets that are magnetic that that would require securely gluing a metal plate to the wall to which to attach it -- such as the kind of metal plate often used for 911 numbers:

https://www.amazon.com/Hy-Ko-Products-911B-Address-Numbers/dp/B00DDVTFMM/ref=asc_df_B00DDVTFMM/?tag=hyprod-20&linkCode=df0&hvadid=193154321880&hvpos=1o5&hvnetw=g&hvrand=11618609397439564423&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9011011&hvtargid=pla-309744497906&psc=1

or you could settle for a towel bar attached to the interior of your bathroom door..

Other options could be to use one with only a small bracket (wide mouth canning jar) that swings out from the wall.

https://www.google.com/search?q=decorative+swing+as+towel+rack&source=univ&tbm=shop&tbo=u&sa=X&ved=0ahUKEwjz5JCz1rDkAhWCUt8KHYL-B_wQsxgILg&biw=1344&bih=620#spd=14677656640257867637&spf=1567377024088

You could create a whimsical "swing" to be hung from the top wall plate. Attach your towel bar to board several inches wider than the towel bar and drill two holes in a board. Use nylon cord with the ends fed thru the two holes and knots on the towel rack side to create your rope for your swing. The board will be usable as a shelf and will stay in place if you add a vertical board to the back of it to place against the wall.

I have on that was given to me as a yard decoration with one cord and two holes and with the back board the shape of a child on the swing and a front board in the shape of her legs. the original board had two holes and two small flower pots set in it. I replaced the board with a piece of treated deck board for use as a shelf. Forego the child's legs hanging off the front of the swing and add your towel bar beneath the shelf instead.

https://www.amazon.com/ANZ-Products-Premium-Hanging-Floating/dp/B07G7G8MKF

Coat rack.