While I have installed or custom made various improvised bathroom grab bars in the past as in this example:CLICK LINK
only today did I learn commercially available vertical grab poles exist and are available for sale at specialty stores or even light duty versions at big box stores.
A perfect choice for those difficult locations where drilling or wall attachments are not practical.
Here is one source to consider: CLICK LINK
Of course, if the ceiling is more than 8 or 9 feet tall the options are once again limited to some variation of custom made alternative installation.
Kattermann's Handyman Blog 